How is a superhero movie supposed to appease die-hard fans while appealing to a new audience? Easter eggs and lots of them.
Sometimes its as simple as a name-drop or allusion to other heroes. Dr Strange, for instance, got his shoutout in Spider-Man 2 way back in 2004. Directors also love to give a nod to earlier iterations of the franchise -- a well-placed ringtone in 2008's Iron Man plays back a MIDI rendition of theme song from the Iron Man animated series that aired in the mid 90s.
References to the source material make appearances as well: Stan Lee's Fantastic Four cameo as canon mailman Willie Lumpkin, for example.
The strangest of these involves 1978's Superman: The Movie. A young Lois Lane points out a teenaged Clark Kent to her parents, played by Kirk Alyn and Noel Neill. Neill and Alyn are best known as the Lois Lane and Superman from the 1940s serial.
Of course, most of these Easter eggs are hard to catch unless you're looking for them. Say what you will about big, loud tentpole movies, at least the better superhero films are smart enough to reward multiple viewings.