When Tara was first introduced, she was shy, introverted, withdrawn, barely ever even speaking when she should have. By the time of her death, she was a more powerful witch, though, obviously not in Willow's league, outspoken, and not afraid to stand up for what she believed was right, even to the point of breaking up with who was almost certainly the only person in her life who had really loved her. If she had lived through Season 6 and on into Season 7, I believe that she would have sided with Buffy in her belief that Spike deserved a chance to prove himself and would have been a calming force during the mutiny against Buffy's authority (just my opinion on those last two, of course) but above and beyond all of that, if she had survived into Season 7, she would have performed one service of incalculable value to the show.
She would have kept the Willow/Kennedy atrocity from ever happening.