That's a bleak assessment... I liked Iron Man 3. It was, at least, a massively better movie than Iron Man 2.
I'd say this Avengers is worth seeing in IMAX, but it's not on par with the original. There are too many moving parts, too many characters they're trying to deal with all at once. It's the entire original Avengers, plus they have to make time for Hawkeye who was brainwashed last time, plus two new characters in the Maximoffs, plus Bettany's character.
The villain is a great concept but not terribly well executed - he just doesn't get enough time to develop and is always just doing things. The benefit of Loki in Avengers was he had an entire movie to be set up (also, he wasn't CGI). The other issue was he was a bit too wise-cracky, and combined with the lack of development time this just made him seem less significant as a threat.
Is this a spoiler thread? There's no spoiler tag in the title, so...
All in all it's an entertaining flick and I'll watch it again. Had a bunch of clever lines I laughed out loud at. But it was too unwieldy to manage for Whedon this time around.