On it's own ME2 is incredible in terms of character dialogue, art direction, sound design, and gameplay improvement overhaul. I would argue that from a lore and world building perspective it was a failure to commit to previously laid out ideas about the Terminus systems being a second set of galactic species and powers.
We could have laid the foundation for getting them to work together with council space for the Reaper invasion instead of the actual plot of ME2.
I absolutely love ME2, but I often wonder how they could have reworked the second game to make it supply more ground work for ME3.
Then again, I don't think any amount of plot would have saved ME3's two year dev cycle. It was always going to feel like an incomplete game, rushed. I often hear people blame the shortcomings of ME3 squarely on ME2 and I just don't agree. Could they have done a better job? Absolutely. It likely wouldn't have made the ending any better, and unless you consider bringing the Reapers into the mix in the second of three games, the Reaper war would be crammed into the third no matter what.