I just recently introduced the #worldbuilding tag. It's a baby still, but I hope it grows. A couple people have started posting questions and asking for advice about their worlds, and I've been making "Worldbuilding 101" posts in hopes to get some interest in it and encourage posting.
Our choices do matter to our experience, though. If I started telling everyone on Hubski to go fuck themselves and spammed pictures of goatse I'd end up muted and hushed by the whole damn site pretty quick. All the little choices we make affect our lives in different ways. Other games actually do what Mass Effect pretends to do here with dialogue. I've been playing a lot of Baldur's Gate (I & II EE) lately. Now there's a game where most of what you say matters. The same can be said of Torment, or of KoToR or Fallout. Most conversation-choice heavy games, the ones I"ve played anyway, tend to tie conversation choices into the plot or into the outcome of particular scenarios to a much greater degree. You get a handful of choices like this in Mass Effect, but the vast majority of your conversation may as well not happen as far as the plot is concerned. You won't change much by just mashing your way through it. Like in Baldur's Gate right now I've actually got two games going. In one my party is super evil and I'm constantly fighting guards or bribing priests. In the other I occasionally do something crazy but I've been mostly heroic. They give you two significantly different experiences of the game. Characters say different things to you, prices are different, some people who would aid you in one situation will attack you in another. Even your stats affect dialogue. I mean, come on. It's only like 4 times as much writing. ;)
I must admit, I haven't played each of the Mass Effect games more than twice, and both times I've played as Paragon, so I have as much idea about the difference as a person who's read about it on the game's Wikia. Moreover, I've never replayed RPGs with different morality because I don't get the kick from it. I've never played Baldur's Gate and I barely touched Torment: Planscape. Perhaps, you're right to say that Mass Effect is not on the level when it comes to dialogue affecting the world. Maybe it wasn't designed as such. Maybe it was designed to tell a story which has more rails than, say, Baldur's Gate. Maybe it was designed to tell a story, closer to a book than to free-roam RPGs. As such, it succeeded, and as such, I appreciate it. Don't judge it by what it's not: it will never become it. That being said, have you tried Arcanum or Pillars of Eternity? Those are the kind of games you might very well enjoy.