I'm not sure it is that Spiderman is inherently better suited for a series than other superhero/comic movies as much as they haven't messed it up yet. Batman and Superman both started out with lots of hope for the future, but started to fizzle out at 3. At least Spiderman has thus far avoided two major pitfalls: No changes of director, and no comedians as villians. So I do hold out hope that Spiderman can keep going longer than the others. I guess I'm just would of perferred for them to have used at least one of the future villians they introducted in 2, instead of the seeming "easter eggs" for comics fans to notice.