See, here's the deal. When I play through a game with a morality system, it's generally:

First time: I do what I want (usually tends towards good, with a bit of jackass when it gets me more dough, or when it really calse for it, i.e. the beginning of Tali's trial ("Oh, by the way, your dad is dead").

Second time: Evil, to do all the jackass things I didn't my first time through.

Third time: Good, to be worshipped.

Now, my first playthrough, I got kind of fucked over. Because ME's morality system is such that to be able to convince people in late-game, you have to had stuck with one alignment more-or-less the entire game. So, while I wasn't a Renegade when unnecessary (healing that Batarian, stopping the kid from signing up, etc.) I did do it sometimes when the person I was talking to was a huge dickface (That guy in C-Sec for Thane's loyalty mission, the Chief guy at Tali's trial). Unfortunately, this meant that later in the game, certain options were unavailable to me (ex: Miranda v Jack).

So, my paragon and renegade playthroughs, things went/are going pretty well, but my main point is this:

Mass Effect's alignment system does not allow proper roleplaying. If I do not choose one alignment 95% of the time, I can lose a teammate's loyalty. And sometimes, even when playing as a pure Paragon, I get jackass points for a line here or there. So, how am I supposed to roleplay when I have to keep considering "will this affect my alignment too much?" This, along with Mass Effect's dialogue wheel's setup (Top = Paragon, Bottom = Renegade), can lead to people not roleplaying, but instead just holding the stick facing top-right, and rapidly tapping X (like my friend does for romantic prospects).

In summary, Mass Effect needs a separate category for some Charisma-like stat. Otherwise, true roleplaying will just not be feasible.