Tightened the graphics up,
Improved the budget and cast,
Had a mountain of top notch side quests,
Added a boat load of memorable characters,
Refined the shooting,
Added the awesome Interruption mechanic.
Scraped a lot of the exploration and sense of scale,
The main plot was very poor in comparison to the first game,
EA's "familiar and safe" policy could be felt (giant robot final boss anyone?),
The shooting was slicker but not as much crazy fun (biotics were near useless most of the time),
All but mandatory DLC up the backside,
Mac "Humans are super duper special" Walters took the helm as the lead writer,
Brutally murdered the RPG mechanics,
The series changed from a Reconstruction of the sci-fi genre to normal sci-fi silliness (oxygen masks in the open void of space...for girls only!),
Forced the player into stupid choices, made grey issues into very binary ones at times,
Tone: DARK AND EDGY: ARE WE COOL YET GUYS!?
I came away from ME 1 blown away by the finale. Very satisfied.
I was laughing at the silliness of ME 2's "conclusion". I felt like I'd achieved nothing other then ruining TIM's day, when I would rather have just ignored TIM right at start.
ME2's plot was a side quest I just wanted to ignore. The whole Cerberus thing was forced and shoehorned in and put me in a bad mood from the start.