While the first three years of the show make it feel like some characters' arcs have taken too long to develop, viewers have to remember that all three took place back to back without any time passing at all between seasons. Even within seasons, not much time had passed. Some estimates online are that the whole series up to now has happened in a matter of a few months. Arlene's still in her first trimester towards the end of season three.
So for those who complain about Tara's development, well, pick any three months out of your adult life and figure out how much you changed significantly in that time.

And as far as she and Sookie not feeling much like best friends, best friends often drift apart a little when just one of them is suddenly in a new and passionate relationship. In their cases, they both fell in love during a short span of time; further complicating their lives is everything around them going batshit crazy after the revelation of all the supernatural creatures they never knew existed.
Also keep in mind that this is probably the first time in their lives they've ever kept things from each other. Sookie purposefully works hard not to read her loved ones, and with Tara, there wasn't much need to anyway, considering how close they were growing up. Once things become chaotic though, Tara especially begins to keep secrets from Sookie (shagging Sam being just one example).
I loved the sequence at the end of Season 3 where Tara, who arguably is one of the smartest characters in the series, has her moment of realizing just how warped everything around her is. Her cool and calm manner when she greets Sookie in Season 4 shows her love for her friend, but it's not a desperate kind of love anymore--she has a life that does not depend upon nor revolve around the Stackhouses now. For Sookie, only a day or so has passed; for Tara, it's been a year in which she has discovered new parts of herself.
It makes a great deal of sense that Tara is a lesbian, or at least bisexual. Her first great crush was on Jason, whom she could never have. Her sex with Sam was primal and raw, but she couldn't develop the emotions for him that could bring her happiness. Eggs--well, that was complete chaos. She once again makes a bad sexual decision about Franklin and ends up emotionally and physically brutalized. Her one great love her whole life has been her best friend, Sookie, and when they both begin making bad decisions about men, they begin to have what is apparently their first big fights. I've watched similar things happen to friends who were seemingly hetero but then began to accept their lesbianism and their attachments to their best friends.
Or maybe Tara is not a lesbian now, and this is another way for her to act out her sexual needs with someone she's destined to not be able to stay with. She's already lying to her partner about who she is and what her past is, so that could fit as well. Even with her newfound growth, she's perhaps still living out those old patterns of chaos. It's certainly realistic.
Whatever the case, I have a deep platonic love for Rutina Wesley. Some of her scenes in Season One with Tara's mother still break my heart when I watch them.