I had fun, perhaps because I am just a bit too fond of Robert Downey Jr.'s Iron Man. (I think the first "Iron Man" is still my favorite of the Marvel movies, though.) Mark Ruffalo as Bruce Banner was pretty good, too. I liked seeing that female SHIELD agents are allowed to dress professionally. For the most part.
I did not really find the ending satisfying. For one thing, was it really necessary that the evil alien soldiers were drones, who fell over when the mothership was destroyed? This just smacked of cartoon logic to me, where it's OK to kill tons of foot soliders as long as they're robots. It emphasized that we didn't see any civilian casualties (perhaps to avoid an R rating?) and the movie just didn't really generate an authentic sense of peril.
There were a few lines that made me roll my eyes and want to go "shut up Joss Whedon". Like when Hawkeye and Black Widow were bantering about Budapest. Thor's bit about "oh thank goodness SHIELD got my girlfriend out of the way" felt like an insert--- on the other hand, pretending it wasn't an issue wouldn't work either. Threatening Loki personally only works if you carry through and focus your efforts on Loki during the battle. Which they did not.
Also, WTF is up with flying aircraft carriers that have only N+1 redundancy? Better to have six rotors and be able to lose two (or three.) I mean seriously, folks, do some engineering analysis. Not to mention that SHIELD seems to have piss-poor authentication protocols for release of weapons.