She is the quintessential woman every man wants.
Shes:
1. Strong and independent, yet willing to show her feelings to anyone she feels close with.
2. Shes able to speak her opinion in an educated way, without coming off as a b*tch. ((Example, Miranda. "I am made to be superior to ever other person in the galaxy and yet im still the most useless person to join your squad, and i disapprove of everything that isnt 100% by the books.".))
3. You already have a relationship with her before the game starts. ((lets face it, shes the nerd of the group. And a decent portion of video gamers are "nerds". I myself included, so we have see a sort of kinship with her because of her disposition.))
4. Shes mysterious. Because her face is hid, and she has a well proportioned body ((DAT A**)), she is the "Every Woman". She can look however we choose for her to look, and nobody is wrong.
5. She is the most vulnerable. The slightest nik to her suit will cause an infection. She is physically frail, and that a huge part of who she is. Shes the "damsel in distress" we all want to rescue, but to the extreme.
I could probably come up with more reasons if i were to really think about it, but those are the first ones i can choose of the top of my head.
I myself, am a Tali fan. Shes a great character, in a great series.