This was my review, back in 2012, of Marvel's The Avengers. (Some spoilers in what follows.)
In preparation for seeing Avengers: Age of Ultron, I watched the movie again a couple of nights ago, and I now tend to think I was a little too harsh when I originally reviewed it - perhaps my expectations or hopes were too high at the time.
Watching it again, I still thought the closing battle was somewhat anti-climactic, as the Chitauri were just so incompetent. It never felt (to me) that they were a real threat to the good guys. Consequently, that battle scene became (to my taste) too much a showcase for what the individual Avengers could do, when pitted against a vast alien army, and not an emotionally convincing case of bravery and resourcefulness overcoming the odds. This is despite the fact that the Chitauri keep coming in seemingly limitless numbers.
So, that criticism remains. Of course, it depends on what you want. If you basically want to cheer for the heroic characters - or your favourites among them - as they cut loose on a faceless enemy horde, then this won't be much of a criticism.
I remain impressed by the way all the main characters were built up, shown as badass, and (at least in most cases) kept close to the original source material. Most of the performances were excellent - e.g. I was much more struck this time by Mark Ruffalo's performance as a somewhat skulking, enigmatic Bruce Banner.
Loki's confrontations with the individual heroes were impressive, done in a way that provides good showings for each of them but also for him as the villain (even when he gets smashed around by the Hulk near the end, he is shown soon after picking himself up almost unscathed and declaring that he could really do with that drink Tony Stark offered him at the start of the battle).
Good, fun stuff - and I'll report back soon on Age of Ultron.