Prior to the 1960's, so-called "anniversary issues" typically weren't acknowledged on a comic book's cover or, if they were, the publishers were fairly low key about it. A good example of the more subdued anniversary cover would be something like Superman #100 (1955).
However, when Marvel Comics entered the picture (a publisher few would describe as "low key" or "subdued"), anniversary issues went from non-events or austere cover galleries to bombastic, star-studded celebrations. In fact, when it came to high wattage covers for anniversary issues, Marvel easily outpaced DC Comics, who could never really muster the same kind of energy for their milestone issue covers.
My favorite Marvel anniversary covers were the ones featuring the title character (of course) along with a blizzard of supervillains and (occasionally) supporting cast members. Yeah, the stories inside were often a let-down (as the promised horde of supervillains often turned out to be only robots or mind game hallucinations), but the dynamite covers were usually enough to overcome the disappointment.
Here's three of the very best (click for a larger view):
This "Everthing but the Kitchen Sink" style of anniversary cover gradually fell out of favor, especially after the modern trend of celebrating "anniversaries" every twenty-five issues made the whole practice a little less special. Ah, but for a time, those 100th or 200th issues were something so special, they demanded to be bought!