ME2 vastly expanded the universe of mass effect from the very bare bones level of the first game. It makes the reapers into more than vague robot threat that kills the universe every so often. It established other races as more than basic caricatures. You can keep the basic narrative intact without it, but you lose the sense of payoff in 3 without seeing krogan as a dying race, geth as a sentient race that deserves equality, and the truly desperate nature of the nomadic quarians.
3 was pretty good until the final ending that was clearly rushed in establishing the full reasoning behind each choice. Yes it had multi-player tacked on, but it was clearly a rushed effort and cutting it wouldn’t have fixed the story. The multi-player is also the best coop gameplay I’ve ever played and nothing has came close to the feel. You’re problems with 3 and other Bioware releases seem directly related to the broad direction EA was forcing everyone down.