MP2 maybe didn't do much new over MP1 except have some much larger areas. but MP3 did quite a lot more I thought. for one thing, there are other people including other bounty hunters that you fight alongside, it plays a lot differently with the wii-mote, there are "action scenes" where large scale things happen with you still in control (can't remember exactly but it was fairly impressive I thought). I don't know if I can say it's a great game or not, but it specific DID do a lot more bew things than MP2 did. (although now that I think of it, MP2 did that whole shadow-realm thing didn't it... maybe that's what you mean).
and BL2 vs BL1 is almost not a comparison. BL2 is such a massive improvement over BL1 it's not even funny. BL1 has maybe better gun mechanics and feel and balance, but otherwise BL2 is a *phenomenally* better game in so many ways.
I think that's part of how I view games - game feel and balance are more important to me than other factors which is why while I like BL2, I still play BL1 more.
And MP3 does have some cool stuff like the electrical puzzles and the marines at the end