I think there are some perspective issues to be considered.
First, you may find some blatant negative reviews on these boards, but there are lots of examples of constructive criticism as well that I hope BioWare is taking note of.
Most of all, the main reason why many fans of ME1 are reacting with such intense sensitivity regarding gameplay changes has to do, quite simply, with the fact that it was such a successful game in the first place. Lots of people got very passionate about the wonderful ideas that came together in Mass Effect. And inevitably, as they played Mass Effect 2, they confronted it with their own ideas and expectations towards the game. I confess I did the same thing as I played ME2 for the first time. And there were things I thought didn't quite match that vision.
But I also believe that if you accept the game for what it is and take time to appreciate the many wonderful things, details, storylines being presented, you have to recognise this is a wonderful game that, in many ways, actually surpasses ME1. Having said that, I also think BioWare streamlined the game too much, but on the other hand, ME2 is a more focused game, most of its missions serve the main narrative, and in that regard, it's much bigger than ME1. So where ME2 seems to be inferior is because it doesn't feel so open, the universe as wide, because most of the time in ME1 you were wondering around in secondary planets doing secondary stuff that had no real impact on the main storyline.
In conclusion, I think most of the fans that are disappointed are allowing their perceptions and expectations get in the way of appreciating ME2 for what it is. I'm not saying it doesn't have its flaws. Yes many levels are too linear, there is an emphasis on action and less on exploration. And when did the Citadel became a mall? The Citadel is one of the great examples of what ME2 seems to be doing wrong. It's what conquered us all in ME1. We wanted to see more of it, and we don't get the same feeling from Omega or Illium. They seem huge but it feels like the game doesn't quite lets us go too far. The fact that the game is constructed in sections reinforces that sense of limitation, which gives the player a contrived feeling that is unnatural to a certain rpg essence that is to be expected. And if you get too attached to that you may disregard the wonderful details and stories present in ME2.
Basically, we all expected ME2 to be perfect, and we all had different ideas of what that would be. And ME2 isn't perfect, but it evolves in many ways and hopefully BioWare will go the extra mile in ME3. This trilogy deserves it. And, still, for reasons I can't quite explain rationally yet, ME1+2 are my favorite games ever and the ones I always seem to be able to go back without loosing interest. I've played many games that I consider great but these are the ones I don't seem to get tired of playing.