Well, I've already told you I like this fic - and that I have a soft spot for Barret/Tifa - so what else can I say? Let's see...
Tifa seems different in this fic than she is in the game, but I can't call it OOC because the game doesn't give much of an idea of what she was really like just before the game, and I think that this personality would fit with the game.
I do like the subtle romance of it - Tifa respects Barret but the fact that she likes him more than that isn't beaten over the reader's head. Since I can't see these two having a huge passionate romance without a lot of character development, that works for me.
It's a nice fic, although I can't read it without remembering that soon afterwards Cloud is going to come back and this is going to all be forgotten. Ah, well. On the whole I think you did just fine with the challenge.