Ah... Sweet, sweet Tali'Zorah. I'm late to the party. In fact, I'm late to the ME universe. You'll have to forgive me.
Like many of you, I'm sure, I found myself being swept away by Tali's chirpy and perky "See you later!", in the engine room of the SR-1. Tali wasn't carrying a dreadnought-sized chip on her shoulder, like Ashley. She wasn't all emotionally involved with the fact that her mother was someone we were hunting, like Liara. She was just a young Quarian girl trying to make (and take) her place amongst her people. In many respects, Tali was more human than the other humans aboard the Normandy.
The daughter of an admiral might be expected to be spoiled, arrogant and abrasive. The daughter of a father who was never home might be expected to be clingy and co-dependant, looking for the father figure she never had, or have a tough-girl facade that she'd been forced to wear to cover up the fact that her father was never there.
Tali was none of those things. She was bright, energetic, humble and well-adjusted. Confident enough to know when it was smart to be scared. Both cute and adorable but intelligent and mature in equal parts. Surprisingly emotive, for a character whose face you could never see and whose expression you could never read. The voice acting was tremendous. Tali didn't need an Elcor-like custom to convey her meaning and intent. Helmet or no helmet, she was always clear in her meaning.
Even her name engenders the same feelings as her personality. Names are always important, as any roleplayer knows. Ashley gets shortened to Ash. Liara is always Liara, and Tali'Zorah becomes Tali - Short, cheery and friendly-sounding, just like Tali herself.
Like any personality type we meet, whether in real life or in a game, book or movie - Personal preferences will always be the deciding factor in who likes whom.
But by being who she is, Tali hits a lot of buttons for a lot of people - There's a certain 'common denominator' when it comes to attractiveness - And Tali has it in spades.