For the sake of clarification:
The context of a fictional serial adventure shared universe can most accurately be called a milieu. The word continuity, while sometimes used to mean the same thing, is an inapt phrase given the inordinate baggage the term has accrued in recent years. It is more accurate, in this context, to refer to continuity not as an object but a process.
As previously discussed, continuity stands in contrast to consistency. Rather than standing in polar opposition, however, the two processes exist in relation to each other on a subjective sliding scale. Every reader's knowledge and tolerance for continuity varies. While one reader might easily believe a book such as the recent Hulk relaunch to be continuity-light, another reader might see the series as continuity-heavy. From one perspective, for readers familiar with the character, the series can be said to depend on little explicit knowledge of previous stories for immediate comprehension. For this first type of reader, the process by which Hulk fits into the Marvel Universe's milieu could best be described as consistent. Another reader seeing the book in the context of the preceding line-wide Hulk-centric crossover and dealing specifically with the changes to that character's status quo stemming from the conclusion of said crossover might understand these connections to previous stories as intrinsic to comprehension of the new series. There is no objective standard with which to judge.
Certainly, it would be easy to judge the outliers on either side of the scale: most would probably agree that Roy Thomas' later, post-Crisis stories in All_Star Squadron are some of the mostly densely continuity-heavy comic books ever printed. Conversely, few would argue that a book such as Ultimate Spider-Man - specifically designed for the bulk of its run to be friendly to readers generally unfamiliar with the pre-existent Spider-Man milieu - is designed to operate best on a level of general consistency. (Tellingly, the Ultimate line began to flounder the further it strayed from the goal of general, user-friendly consistency and towards a more regimented continuity.) Most books fall somewhere in the middle between these two polar opposites.
Whether or not a story is continuity-light or continuity-heavy - whether it relies less on an attention to rigid continuity or more on a passing consistency with previous stories in its milieu - has no bearing on its quality. The applications of continuity and consistency are neutral methodologies.
It is an observed truth, however, that even the most continuity-light milieus will tend inexorably towards complexity in direct proportion to the duration of publication. Furthermore, continuity-heavy systems within established milieus cannot continue indefinitely without self-correction.
There are two kinds of self-correction processes within an established milieu: proactive continuity and passive continuity. Additionally, within the category of proactive continuity there are two distinct but not incompatible techniques. The first corrections are most accurately grouped under the rubric local, and include such phenomena as retcons (retroactive continuity) and continuity implants. (Wikipedia helpfully identifies three distinct types of retcon: addition, alteration and subtraction. I see no reason to dispute these sub-categorizations.) The second type of correction is the large-scale, or universal correction. Universal corrections usually occur in conjunction with - or, perhaps more accurately, serve as the launchpad for - multiple local and passive corrections. There are no divisions of passive corrections.
A good rule of thumb is that if a milieu correction draws attention to itself in any way, it is proactive, whereas if it merely occurs off-panel without explanation it is passive. The original Crisis on Infinite Earths, Zero Hour and Infinite Crisis are all proactive corrections of the universal type. The correction of the Superman titles that followed Crisis, stemming from John Byrne's Man of Steel, were also proactive (in that these changes were the subject of a specific storyline) and local (in that the corrections primarily effected the Superman family of titles, with other series effected tangentially). Conversely, the changes wrought on the Batman family of titles in the wake of Crisis were mostly passive. There was no character reboot (the continuity implants of Year One, Two and Three came a little later, and had nowhere near the far-reaching effects of Man of Steel), and the degree to which the character's pre-Crisis milieu remained intact into the post-Crisis order did not seem to be dictated by anything other than the immediate needs of individual writers and editors.
Spider-Man's One More Day storyline was a proactive local milieu correction whose end result - the still-ongoing "One More Day" status-quo - has mostly unfolded passively, to the frustration of many readers desirous of a more concretized correction process. This has had two different effects: on the one hand, it enabled the creators to move quickly in establishing the parameters of a new status-quo without being overly bogged down in the immediate explanation of every detail stemming from the original correction. On the other hand, the passive attitude towards the change has created an unfortunate tentativeness to the line, based on the understandable expectation on the part of the readership that the lingering questions behind the correction will be addressed explicitly and not merely, as has happened so far, in media res. To a segment of the readership, it is possible that the "Brand New Day" status quo will not be seen as legitimate until these lingering questions are addressed. Similar attempts to graft local corrections onto Spider-Man's milieu, such as Byrne's Chapter One have proven less successful, and have been passively phased out. Kurt Busiek's Untold Tales was another active correction, albeit not one designed for the purpose of simplification, merely elaboration. As such, its effects have been predominantly passive.
More to follow.