I see EA's greasy fingerprints all over it...
ME1 was a superb game, but with a few annoying features, such as the inventory. Awesome plot, awesome "world" to be in.
ME2 had some great missions, better graphics, better character interaction, but was so stripped down and oversimplified on the gameplay front that it made it (for me anyway), far less enjoyable than it should have been. The rock/paper/scissors concept was taken to far, which meant that some squad-mates in the game were unusable for some missions, unlike ME1, where you could tailor your team to your playing style and still come out intact at the end. For example, in ME2, Tali is effectively useless for any mission without Geth in, and Jacob has a skill set that's effectively useless full stop.
ME3 effectively balanced the complexity of ME1 and the streamlining of ME2 and created a game in which the gameplay works better than ME1 or 2. It also has a few missions that are some of the best I've ever played in any game (Tuchanka for example).
The sad thing with ME3 is that it was quite clearly released a year too early. Some sections of the game feel rushed, there are quite a few noticeable bugs (something I never experienced in ME1 or 2), and some of the missions (Thessia for example) just feel far too short and incomplete.
Then there's the ending.
I won't bother to say anything about the ending, because I can't say anything that hasn't already been said, and probably more eloquently than I could have said it.