I saw IM2 on Friday. As an Iron Man fan for almost 30 years I have to say I thought it rocked. Just about as good as the first. Plenty of action and yeah, they brought in some more characters, but they were all great. Hammer was fantastic - Rockwell did a fantastic job breathing life in Stark's main competition. Loved the extra scene at the end too, though I'm a little bothered that they put it at the very end when 80% of the audience has left. Fury was kind of a big deal in this one, so if you missed the final scene from IM2, you might be wondering who the heck he is.
As for the Avengers, it would be great to see Ultron as the core villian and that's who I have my money on. The Loki/Hulk bit is more true to form, but the Pym/Ultron story is far more interesting and lots of ways they could play it. Also a nice way to bring Pym (and hopefully Janet) into the mix, since they won't have their own titles ahead of time.