Mass 3 was a huge improvement to be sure and probably the best game in the series until Priority: Earth. Yet I think Mass 2's Recruitment and Loyalty missions are that game's best features and genuinely formed player attachment towards the team. So when people say they preferred ME 2 I can only assume this is the reason why because nothing else makes any sense.
That may be part of the reason. For me, ME2 is my favorite because I feel the combat system, despite some hiccups, is at its best. ME3 brought some changes, but I do not feel that they all improved the combat. I feel some of the changes brought regression to the system and some were half-baked or poorly implemented.
I still enjoy ME3 (even more so in multiplayer with friends) but I did not get the same level of "wow" factor that I did with ME2. That includes both story and combat.
ME2's story may be the weakest, but I think it was structured the best. ME2's end game (suicide mission) may be 'urgent' but unlike ME1 or ME3, you never feel like the urgency is being sidetracked by all these secondary missions. Recruiting squadmates, gaining their loyalty, even completing optional missions like Overlord or Lair of the Shadow Broker felt important because it felt like it was all about preparing for the suicide mission, even if it was not.
In ME1 you had to stop Saren. The main story missions leading up to stopping Saren (Feros, Noveria, Therum, Virmire) set the right tone for the story. But all the other side missions just felt like filler/XP grinds since they did not even have a tenuous connection to the main story, so it felt like the story was sidetracked and the sense of urgency was lost.
And In ME3, the individual story arcs felt like they fit in the whole of the story, but a lot of the DLC and half the side content, despite being strong, just did not evoke the same sense that they connected.
But those are just my personal feelings on the matter. Not everyone feels the same way I do.
And for the record, I like Priority:Earth, even though I do not think that the game should have ended on Earth -- or even allowed us to go to Earth at all. Priority: Earth could maybe have been a bit better if you fought Reapers and Cerberus and Geth all at once, like the Reapers were throwing all they had at you to get to the beam. (And by Priority: Earth, I mean No Man's Land and the Missile Battery. The Hades Cannon mission can go straight to development hell.)