I don't give a damn about the mystery. It's never been about 'seeing her face' to me. It's been that all throughout the series, Tali is this curious alien creature. She's a completely non-human entity with enough human traits to relate to. When you meet her, she's on a quest to learn about the world and find something of interest and use to bring back to her people.
The relationship aspect of her character in Mass-Effect 2 was very similar.
It wasn't about the mystery... it was about this curious young Quarian girl coming to terms with her feelings for someone of another species. She's timid, and nervous... and awkwardly flirty. It's amazing. And it has absolutely nothing to do with her face.
Anyone who has ever "genuinely" liked a person knows that you can overlook imperfections in appearances. People will 'get over' her not looking quite like they expected, if they see her. Conversely, people will also 'get over' her face never being revealed.
To me, though, it seems quite caddish to hide her face under the claims of 'maintaining mystery'. The real argument is "Do people want her to be shockingly alien, or surprisingly human? Do people want her to be a thing of beauty, or hideously foreign.'
In the end... people who truly like the character for what she is won't really give a damn what she looks like. They'll love her anyways.