I really don't understand the hate for it.

It's better then IM 3 imo

It's because a lot of it feels more like an Avengers set up than an Iron Man movie. Also Vanko was a bland villain. Hammer was too comical to be taken seriously.

It's still better than Iron Man 3 though.

I enjoy this movie, however I do think it falls short of the potential the ideas it sets up. I will say that I think the first act of the film (all the way up through Tony talking to Vanko after he's captured) is great. The issue is that the film after that while still fun to watch, is basically Tony bumming around his house with a lot of comedy. The main thing it drops is the whole, "If you can make God bleed, people will cease to believe in him...There will be blood in the water, and the sharks will come." idea which was great set-up for basically nothing. And Vanko for no good reason goes after Stark again, even though for all he knows his plan in harming Stark's legacy and image has succeeded. And as he said, he should be just waiting to watch the world consume him. I think it would have been stronger had some "sharks" so to speak actually come. Build that into the idea of how Tony cannot handle everything all by himself in that kind of situation. Like let's say, have one or two sub-villains inspired by the fight in Monaco to take a shot at Tony after his image of invincibility is tarnished. The "nuclear deterrent" is made to look less powerful. And that could be connected to the idea of whether or not he should hand over the armor for mass production and so forth. Tony during this time manages to subdue the threat whilst figuring out the issue with the palladium poisoning. Vanko finds out about this, and that's when he shifts gears to kill Tony instead which he just kind of does here for no explored reason even though he hasn't figured out that Tony's gotten over the poisoning and what not. (With him at first building the tech for Hammer just to create his way of escaping at first like Tony in the original film with the Ten Rings) Naturally there's a lot of unfilled holes here, but I think you get the idea. I'll also say that it feels like the film while still fun to watch lags a bit in the middle, and at times the Avengers stuff gets a little intrusive on what is supposed to be this film's main story and ideas which get watered down because of them.

I was actually more disappointed with Iron Man 3 in an immediate sense on first viewing because of the whole Mandarin ordeal, but as a film in of itself it is stronger in my opinion. It has a well defined and complete arc for Tony and keeping him proactive throughout the film, more fully plays with the thematic ideas it sets up, and has a villain whose motivation as it stands felt a little weaker than I would have liked(the whole getting stood up on a rooftop thing) but was more consistent with what he was doing and why. (I find Vanko more interesting in concept, but I do think they needed to refine what's done with him narratively)

That's not to say I think Iron Man 2 is awful or anything. It's a solid film and a lot of fun to watch IMO. I just think it could and should have been stronger and falls short of the potential it sets up early on. Though I don't think either film has measured up to or surpassed the original. If Iron Man '08 is the series' Batman Begins, I feel like I'm still waiting for its Dark Knight so to speak.

Naturally that's all just in my own opinion.