there are a few things that define an rpg to me.
1) branching story that branches based on your decisions. mass effect is good here, but lets be honest, we all wish that we had better options and more middle ground, and also that we could read what the character would actually say apposed to a 3 word synapses. i want to play a character that doesn't give a damn, and is more or less a accidental/reluctant hero, mass effect gives me pure white or pure black without taking into account prior actions. lets say this person wronged me, in real life i would hit/punch/verbaly abuse them for it, and most people could say that they deserved it. in mass effect i'm given the option of other cheek which is good guy, or hit them with is bad, when in reality it would be neutral. i went on a little long with this one, but you get the idea, mass effect goes a long way where dialogue is concerned, but could do so much more.
2) the stats. i know that d&d the stats get in the way of a good game, but we are playing a video game, and the stats have to be there. in 2, level was an in name only kind of deal and you have no actual choice on characters progressing. in 1 i could chose what and where points went, and i loved doing that.
3) the equipment, i believe that in 1, you could chose a full load out, get better guns, mod those guns, and pick which bullet those guns fired. but in 2, i cant think of one instance of that. even if you class didn't specialize in it, you still got to be able to use the gun/guns. but they basically took that out for mass effect 2, i don't believe there was even a need for me to ever get a better gun.
basically they took the leveling and the equipping aspect out of the game, and made it more about action, and i hated that direction with a passion.
the dialogue trees and the branching path... look at how often Shepard got jerked around, unless there is an option at the end to live with the (good) geth, to join the reapers, than what would the point be? to me they are just getting whats unnecessary down for the trees, without ever making a truly branching story. its nice, but that alone wont hold a game as an rpg for years, than they took out every more complex aspect of the game for a more mainstream run and gun approach, which i despise.