Dark Knight was better than Batman Begins. Hellboy II was better than Hellboy (actually it wasn't but I'm in a distinct minority on that opinion). Mission Impossible III was better than Mission Impossible I (not a superhero movie, but may as well be). X-Men 2 was better than X-Men. Electra was better than Daredevil, lmao. Many James Bond films have been better than their prequels (and James Bond is really as much a superhero as Batman).
Anyway, my point is, superhero sequels, like all sequels, are derivative by nature since they feature the same characters and back story. But, like other all sequels, this doesn't necessarily make them worse. And considering the mediocrity of Iron Man 2, there's really not much reason to think that Iron Man 3 won't be as good or better (especially considering the excellence of The Avengers). Of course, it probably won't be that good. But I'm just saying at least wait for the reviews. 
The whole new batman trilogy sucks, so that's like saying the second turd stinks less. All the mission impossible films are crap and they aren't super hero movies anyway. Electra could join the ranks of of Catwoman as one of the worst super hero movies ever made, and keep in mind Ms. Gardner is from West Virginia, so I'm actually biased in her favor. Any James Bond film without Connery in it is by default inferior just for his absence. Hellboy and Hellboy II are on equal footing in a way, but the first one had lovecraftian monsters and nazis, so that makes it the better film. X-men was better than X2 because it had an actual villian and 3 was a train wreck.
And while we are on that theme name me any franchise, super-hero or otherwise, where the films are still great by part III (Back to the Future and Lord of the Rings don't count, they were pre-planned).
I'm not saying it will be bad, but it defiinately won't be good... or at lest not enough to get excited about it.
And your right, just because it's a sequel doesn't automatically make it bad, but take that, along with the fact that it's a modern Marvel Superhero movie, combined with the fact that it's the third film in an unplanned trilogy means it doesn't have a chance in hell of being anything better than "eh it was good".
Not that DC is doing in better (they are doing far worse actually) but Marvel movies are really bad. They typically have next to no plot aside from the origin film and they pull source from the inferior ultimates universe. Using ultimates back-story is like WB doing a Batman movie, but using the Bat-Mite mythos instead. The X-men films were kind of ok, but you've got to remember that Fox did those. The first two Spidermen were fantastic, but that's because they used mostly the classic story lines and Rami was directing.
The over-use of CG really hurts these films as well. While the "normals" are impressed by the special effects in these films, I feel like I'm watching a video game and that is NOT a good thing. Practical effects or GTFH! Again, go back to the really good ones (Spiderman, and to a much lesser degree X-men) and they used mostly practical effects.... at least in the good films of the franchises.
But this is decidedly a problem with super hero and big action movies more so than other genres. You say that all sequels are derivate, but that isn't true. The Hellraiser and Tales from the Crypt films are all completely different from each other. Indiana Jones films are also unique. Sure Indy is in each one, but that's the only returning character. Not only that, but the location, pacing and general theme of each sequel was different as well. There aren't a lot of franchsises that have pulled it off admittedly, but the way superhero films are setup they are derivate by design. Think about it... typically a superhero says in a single city. They have a set supporting cast that follow them throughout each film, there really aren't any other people in the film aside from the extras, so there's only one thing that ever gets changed... the villian. That's great if it's Batman or Spiderman... they have a ton of interesting villians, but if you are Thor your stuck with Loki... if you are Captain America you are stuck with the Red Skull and if you are any of the lesser heroes.. hell you don't have a memorable rouges gallery at all!