Yeah but when it comes to movies you can ask yourself:
"Was this a good movie? i.e. did I enjoy it?"
or
"Was this a good movie? i.e. does it adhere to main conventions of what's regarded as good quality in movie making?"
ASM2 falls under both categories as a 'no' for me. Avengers falls to heavily under the 'no' category in the second option for me and as such sways my opinion of the first. When the 'no' in the second category is high enough there ends up being a lot people can complain about.
Deus Ex plot devices, unrealistic character development, lazy cg, trope filled story, cliché use of directing to show pivotal plot points, lack of imagination and attention to detail where it matters resulting in reduced feelings of immersion, poor acting (looking at you Thor), ridiculous plot holes, and the ability to still keep everything quite forgettable are all reasons one can criticize movies (among many others).
Comics fall into a category that make it worse due to fandom. We have a comparative source material that is most likely going to be better than the movie. We also have passion and nostalgia. This makes our opinion unbelievably bias from the get go.
Unfortunately I'm a critical cinema buff as well as a comic fan so there actually aren't too many Superhero movies I think are that good. I don't think action blockbusters are usually that good though. Avengers 2 looks like it will be one of the best thus far and I think a huge part is that the villain has more development and we have a better sense of what drives the characters (good or bad). So far in all of the Marvel movies Loki is the only one that seemed like a threat, a villain I cared about seeing. Magneto is another good example. All the other MCU villains thus far have been mostly faceless bland characters that need to be there so there is someone to fight. There's no real passion, motivation, or individuality behind them. In my opinion anyway.
Looking forward to Ultron and BvsS as it stands currently. Dr. Strange maybe when it comes. I can't picture any others not being typical formulaic blockbusters that become surprisingly bland and forgettable surprisingly fast. Here's hoping that's not the case.