Two things, both in Mass Effect 2.
1: Grenades. In the first you could have X grenades no matter what specialization you chose, no matter what level, no matter what. You could always have grenades and you can always use grenades. You could also upgrade them and they were fun and easy to use! Not in Mass Effect 2 though. In ME2 you had to have a certain specialization to use certain types of grenades or any grenade at all.
B: Ammunition. In the first all guns had unlimited ammunition. The only limit you had was based on weapon heat. Lay on the trigger too long and your shotgun, AR, pistol or, sniper will beep at you until the weapon cools. Not a real problem unless you're using explosive or, incineration rounds. Mass Effect 2 uses special weapons like Nuke Launchers and, refrigeration guns. Of course those couldn't have unlimited ammo but why did Bioware take away the unlimited ammo from EVERY gun?! I hated that and I hope Mass Effect 3 fixes both of those issues.