Mass Effect 1 implies a thematic direction and approach to the material that Mass Effect 2 does not actualize.
The disagreement in the community about it all stems back to what the first game was able to suggest about the direction of the story, the presentation of the universe and the goal of the game in general. Which is to say it was a flawed, but good attempt to recapture the so-called "80's synth" feel of good soft sci-fi.
Mass Effect 1 suggested, via its narrative approach, world-building, and non-combat gameplay systems, a whole approach to sci-fi that is pretty goddamn discontinuous from the second game. That doesn't make Mass Effect 2 a bad game. It just has a much different, and much more shallow, focus.
To those gamers who were not invested in the implicit direction Mass Effect 1 seemed to point the series, Mass Effect 2 is the clearly better game. To those who were, Mass Effect 2 is a good game that very VERY clearly does not live up to what it could and should have been and is therefore nothing but a disappointment, no matter how "objectively" better it might be.
This should all be obvious by now, of course.