i think that they definitely fixed a lot of issues in mw2 that were issues in mw1, the riot shield is an awesome tool if you can get coordinated with other team mates, the large maps are nice because people camp less, it's also a more rewarding system as unlocks are achieved quicker, however, i do like the simplicity of mw1 in comparison, i don't think any of the assault rifles perform very differently in mw2 (ie m4 performs similary to scar-h)
the key about mw2 is that it is totally customizable with different perks and weapon combos which help even out gameplay over all meaning a person can pick up a copy and after a few hours compete with generals (well almost) which means more people will play online, which means good ratings for infinity ward, i did really enjoy the campaign in mw2 more, more twists and turns, made game play more interesting, spec-ops is a cool mode too