The Sela thing should've really been more deeply explored overall.
I wish it had never happened in the first place. It was a lame, gimmicky way to bring back an actress whose character had been killed off. It didn't serve any real story purpose. And Denise Crosby looked ridiculous in that hideous Romulan haircut.
Well, I'll meet you half-way on this. I liked the way they brought back Crosby in "Yesterday's Enterprise" but the way, and reason, she was brought back in the Redemption episodes was, a bit, gimmicky. And, IIRC, it was done because Crosby enjoyed coming back for that one episode so much so it seemed to be kind-of stroking her ego a bit when she didn't deserve it after she left the show. On most shows when you break a contract and leave mid-season the show killing your character off is the creators' way of saying "Fuck you. Sure, try and be a big star because you can't come back here."
It's fairly common for shows to kill off a character or at least assassinate the character's, er, "character" to make it almost impossible to bring them back. (See: Kutner,
House, John Kelly, NYPD Blue.) But the fucked-up thing is that in Star Trek being dead isn't enough to for a character to be "gone."
So Tasha got an "out." Crosby got an "out" to correct the "mistake" of her leaving the show a dozen-or-so episodes into the first season. (

Really, Denise, get over youself!) And Crosby's been trying to shoe-horn her way back in to Trek ever since.
Now.
I enjoyed how she was brought back in "Yesterday's Enterprise." It was an F'd up situation enough that she could concievably "exsist" again and, well, it was a damn fine episode. I also admit the reason why she was brought back in Redemption was stupid and the use of her was stupid but I'm a sucker for time-twist mind-fucks so I loved the idea of her going back in time from an alternate "present" to the "real past" where she caused influence onto the "real present", bonus points for the mind-fuck that that alternate "Picard" probably caused the whole mess to begin with by allowing Tasha to go back in time.
Again, I'm a sucker for time-travel twists like that so I enjoyed the hell out of the way Crosby was brought back in such a manner. I just wish it was done a bit better, maybe they could've found her/recovered her as an aged Tasha and she dies over the course of the two-parter as the one who ends the crisis, giving Tasha a meaningful death.