And here's why:

It was a sequel that felt like the logical continuation of the first, the feeling was the same and that's always a great thing. It was fun to follow those characters around some more, especially Downey Jr. was, once again, so full of sarcasm, arrogance, but also sudden traces of vulnerability that makes it so easy to cheer for this hero, despite of his "better than you"-attitude. To have him, Jackson and Johansson around one table, and playing these characters, gave me a bit of a geekasm. Rockwell and Rourke were excellent bad guys and worked great together, as different as they were.

Now, after the pretty great action sequence at the Monaco race tracks, the film gets a little too chatty and the middle part has no real action to offer except for IM and Rhodey beating the shit out of each other to the tunes of Daft Punk. There also were a few scenes that could have needed a little more explanation. Like how Rhodey could just come and fly the suit so easily after we witnessed Stark's rather catastrophic first steps in the first film.

What saves the film through that somewhat bumpy part is the humor, which does not only come from Stark but almost the entire cast. Had a few really big laughs there.

The movie finally gets en par with the first by delivering a far more spectacular and fun showdown. The Black Widow scenes are as cool as Kick-Ass' Hit Girl recently.
The after-credits hint at the Avengers showing Thor's Hammer in a New Mexican crater (and Cap's shield showing up earlier already) makes me really look forward to the next films and particularly the gathering down the line.

So, very enjoyable, if you ignore a few flaws. Great blockbuster entertainment.

8 out of 10 donuts