My two cents:
My guess is people who are disappointed by this were expecting a totally new and crazy gaming experience, and I don't think that the average gamer is going to find that in Gears. Ultimately, you run around and shoot people. It's a shooter, that's what you do. At its core, that's a gameplay element that has been innovated on, but it takes a certain type of gamer to really enjoy and appreciate that (the more console/FPS experience you have, the more you appreciate this game, imo). Taking cover, flanking, chainsawing, and a fairly intelligent enemy AI all make for a new and entertaining experience.
This is a very good game. Is it revolutionary? Debatable. Definitely not in the sense that it's clearly one of the best things to ever come out on a console, but it's still a very solid product. I don't think that it pushes the boundaries like a Gran Turismo, Metal Gear, Grand Theft Auto, Resident Evil type game does/did, but that doesn't mean it's bad in any way. It does a lot of things right, and belongs up there with great games. I just don't think it's legendary.