I have to agree with Angmir; I don't dislike Tali and I think it's a bit of an over-reaction to extend the behaviour of some talimaniacs onto the character, however I do see Tali as more of a hero-worshipping character (and if I played a female shephard, I wouldn't romance Garrus either for pretty much the exact same reason).
Also, I don't understand the rationale between sexual encounters between differing species. Hell, I'd conspire to bring Tali and Kal'Reegar together, and I'd conspire to bring Garrus together with someone else, but jumping into bed with them just seems patently ridiculous to me personally. I mean, it seems to me that Tali was literally dying to get together with Shephard, and frankly I find that a wee bit disturbing.
I like Miranda because:
a) She's hot (Yvonne Strahovski). To be brutally honest I think people who say that looks don't matter are somewhat fudging the issue. It may not be the key ingredient to the equation, but if she isn't attractive enough to gain your attention in the first place, it isn't going to happen.

She seems to compliment Shephard's capabilities (i.e., not more of the same... this could be because I usually play a Vanguard or Adept... but she's principally an intellectual sort of character in my opinion)
c) She's obviously very intelligent. A lot smarter than Shephard in any case.
d) She's ruthless. I admit, I find a somewhat dominant personality (well someone who isn't afraid to go after what she wants) to be a 'bit' of a turn on.
e) She's sensible -- she tries to not let issues distract her.
Probably more than that too, but that's what I've identified so far.