I find it slightly disconcerting that there are people out there who may not understand the appeal of Hawkeye.
It is almost certain that the forthcoming Avengers film will get Hawkeye exactly wrong. From all indications - and from the brief preview of the character we got during Thor - the Hawkeye who joins the filmic Avengers is a SHIELD agent whose speciality just happens to be the bow and arrow. This is certainly in keeping with the Ultimate version of the character but - again! - that wasn't really Hawkeye.
The reason why this is important is that Hawkeye isn't just a character in a garish purple costume who shoots a bow - his personality is his defining trait. Meaning, his attitude, his chutzpah, his arrogance and matching insecurity. When you give us a character who is a SHIELD special agent who has spent years training in the military with all the baggage that entails, you've warped the character beyond recognition. Hawkeye may be relatively obscure next to the likes of Captain America and the Hulk, but that's all the more reason to make sure his personality is well defined. I'm sure movie Hawkeye will crack a few jokes and maybe be impulsive, but the movie's very premise precludes Hawkeye being anything actually resembling the rebellious SOB he is in the books.
Or, to put it another way: the context of the movies being what it is, the basic dynamic between Captain America, Thor, and Iron Man should remain largely unscathed (and the Hulk, being a slightly disconnected wildcard in the comics, should come across well as long as he remains essentially a loose cannon). But Hawkeye's hook is almost completely effaced by the conceit of the Avengers being a super-powered special-forces team assembled by the government. SHIELD isn't the kind of outfit where an ex-carnie can waltz in, tie-up the butler, and sweet-talk his way into the major leagues.
Who is Hawkeye? If you haven't read many Avengers comics, you might not have a good handle on his character. After all, he has never been able to support a solo book, he's never been a particularly prolific guest star, and he hasn't really done much of anything in the last ten years other than die and come back as a ninja (sigh). But it's absolutely vital to understand two things about him: 1) he's not Green Arrow and 2) he's kind of an asshole.
The first point might seem obvious, but it bears further scrutiny. Green Arrow is a character who, for large portions of his existence, has had very little reason to exist. Go back and read any Golden Age Green Arrow story, any 1950s Green Arrow story - even Jack Kirby's very weird Green Arrow stories. He was a mystery man in the Batman mold, complete with expensive toys and a youthful ward. Nothing at all original about him. Then in the sixties, they hit on the great idea of making him a lefty, and that characterization has stuck ever since for better or for worse. The post-Silver Age Green Arrow is a memorable character for a number of reasons: he's a principled individual who is nonetheless a callow heel in his personal life; he's a man with a deep awareness of the insoluble complexities at the root of crime who nonetheless regularly uses violence as a means of conflict resolution; he's a rich man who feels a great deal of guilt at his life of privilege. These are all very interesting hooks, and most good Green Arrow stories have used this template as their starting point (also, it should be noted, any number of terrible preachy faux-pretentious stories about Eastern religion and super-spy espionage claptrap, but whatevs).
The one thing that Green Arrow hasn't really been very good at since the late sixties was actually being a super-hero. Sure, he wears a costume and fights crooks, but the moment he started talking about fighting the causes of problems like drug abuse and poverty and dealing with issues on a street-level basis, he stopped making sense as a dues-paying member of the Justice League. Sure enough, although he remained in the book throughout the sixties and seventies, creative teams since then have faced an uphill battle in terms of providing a convincing rationale for someone like Green Arrow to be on the team. By the time the eighties rolled around most writers gave up trying. Green Arrow remained a mostly inactive member of the League - his history with the team was never erased. But there was always tension between the (real-world) fact that Green Arrow was a big enough name that he could never be cut out of the franchise entirely, and the inescapable conclusion that the character himself often wanted very little to do with fighting world-beaters and alien armadas.
Hawkeye, on the other hand, doesn't really work outside the context of the Avengers. People have tried, certainly: he's even developed a small but respectable solo rogues gallery, mostly composed of people from Clint Barton's past who have a mad-on for whatever reason. But he doesn't have any projects, any real reason to operate, outside his team. He doesn't have Green Arrow's social conscience to propel his characterization. Hawkeye grew up a poor orphan, a boy who literally ran away to join the circus and learned everything he needed at the feet of some extremely dubious characters. For Hawkeye, being an Avenger is the best job in the world, certainly more than almost anyone else in his position could ever have dreamed of accomplishing. Whenever he's left the team - to have his own "hard traveling hero" moments, or to gain perspective on whatever recent setback he suffered - it's always been temporary, and often when he's left the main Avengers group it's been for the purpose of founding or joining another group, such as the West Coast Avengers or the Thunderbolts.
But if we've established that Hawkeye doesn't work outside of the Avengers, we still haven't ascertained exactly why he does work in the Avengers. The answer to this question relates back to my second assertion: Hawkeye is an asshole.
On the most elementary level, this is Group Dynamics 101: if you're writing a group - any group - you need to have one member who is excessively abrasive and undeniably unpleasant, someone for the rest of the group to spark against and to incite interpersonal conflict. But there's more to it than that. Hawkeye starts off as a wild-card, a former criminal who bluffs his way onto the Avengers by breaking into the team's headquarters and tying up the butler. He becomes part of the famous "Cap's Kooky Quartet" incarnation of the team, alongside Captain America, Quicksilver, and the Scarlet Witch.
At the outset, Hawkeye didn't get along with anyone. He resented Cap's experience and command, he took an instant dislike to Quicksilver's officious egotism, and insisted on putting the moves on the Scarlet Witch even though it made her brother see red. Remember: Hawkeye started out as a crook. He was tricked into theft and espionage by the Black Widow, who was using her sex as a means of controlling the archer. (This was back in 1964, mind you!) He barges his way into the Avengers based on nothing more than his facility with a bow and arrow and an absolutely enormous pair of brass balls.
This has been the one constant of Hawkeye's character from his very earliest appearances. How else do you think that a normal dude, with just a bow and some fancy trick arrows, could ever actually believe he was worthy to stand next to the likes of Thor, Captain America, and Iron Man - a Norse god who can level mountains with a throw of his magic hammer, a super-soldier who fought in World War II, and a brilliant engineer who was able to build the most powerful suit of armor on the planet out of old car parts and transistor radios while bleeding to death in a bamboo shack in Vietnam? The answer to this question is that Hawkeye gets to stand next to the most powerful beings on the planet simply because he's earned the right to be there. He simply doesn't quit: even after all these years he's still got something to prove. He'll always have something to prove. He covers it up with jokes and an attitude, but he's got an absolutely indomitable will, and alongside it a willingness to do anything - even cheat! - to win the day.
His relationship with Captain America is one of the most complex in comics. Hawkeye has essentially built his career out of trying to impress Cap. Biologically, Cap isn't that much older than Hawkeye: he's hardly a father figure, more like a slightly older, more responsible brother. The difference is that whereas Hawkeye spent his late teens and early twenties working as a carnival roustabout and falling into a life of petty crime, Cap spent his formative years fighting Nazis. Captain American came out of the ice prematurely old: he may have only lived twenty-some years, but he had spent four years in a war, seeing his friends cut down by the hundreds and witnessing some of history's worst atrocities firsthand. He came out of suspended animation ready to fight and lead, disciplined and driven. Here comes Hawkeye: brash, loud, uncouth - talented and yet profoundly irascible. Hawkeye knows he isn't half the man Cap is, not yet, but he will be damned if he'll ever tell him that.
And so even when Hawkeye grows and changes, matures into leadership roles as the chairman of the West Coast Avengers and later with the Thunderbolts, he can't shake that little voice in the back of his head constantly comparing himself to Captain America. He will never cease to want to impress Cap, while at the same time resenting himself for still striving for Cap's approval. This is something that Rick Remender definitely understands in Secret Avengers: no matter how seasoned and experienced Hawkeye may be, he still gets a little torked around Cap, he acts out, he mouths off. Kurt Busiek got it, too, during his run on the Avengers: Hawkeye was the leader of the West Coast Avengers for ten years but the moment he was back on the "real" Avengers with Cap he couldn't help but feel inferior again, like that kid jostling with Cap for leadership of the team back in the mid-60s, spouting off like a hothead besides the fact that he damn well knows better.
But despite all that, there's no doubt that - of all the Avengers - Captain America and Hawkeye are probably the best friends. Cap, Thor, and Iron Man are all close, no doubt, but it's the kind of closeness built by respected professionals after years of working on an intimate basis. They still all have their separate lives and adventures - it's hard to imagine them relaxing together in the same way that some of the more tertiary Avengers do. Brian Michael Bendis - of all people! - captured this dynamic quite well in the Avengers: Prime mini-series. When they're fighting side-by-side, they are close as any three people can be - but when the fighting stops and they have to interact as people, their speech can be a bit stilted, their small talk tentative. They're not quite sure how to shoot the shit with each other. But you get the idea that, all things being equal, Hawkeye is one of Cap's closest real friends - right next to the Falcon, Sharon Carter, and Nick Fury (although, maybe not Fury for a few years since Fury's been written as such a doggedly unlikeable character), and probably more so than even Bucky (whose relationship with Cap is even more complicated and ambivalent than Hawkeye's). They've got more in common than just about anyone else - two senior Avengers who rely on little more than skills and their wit to allow them to hold their own next to the most powerful beings on the planet.
This is why Hawkeye is such a wonderful character: he's a hotheaded, arrogant, completely insufferable prick who nevertheless manages to back up every word (whether he's actually bluffing or not, you'll probably never know). He manages to hold his own in the Avengers through sheer force of will: he's always said he belongs there, and people just believe him. He's always going to be smiling because he knows, on some level, he's basically living every kid's dream: he gets to play Robin Hood and save the world, and sometimes he even gets to date the most beautiful women. What's not to like about that? He loves his job, and you would too.