I find the talent stumps to be a miserable, steaming pile of wretchedness. I didn't like it when I tried it out after patch 5.0, and I liked it less after I tried playing MoP again over the holidays. I play disc priest and rogue, and my experience with both classes was that I lost significant abilities, and then got to decide on a few I could get back, all while being told by Blizz that I now had "more choice", which was a bald-faced lie.
While there may (emphasis on the may) have been a single optimal raiding dps build with the old, there is (potentially), or at least there was, a lot more to the game that just raiding - you could have a build tailored for 5mans, questing, different flavors of pvp, soloing old content, etc. As for those who say, "you can just change talents on the fly" I would point out that: 1) I have a smaller choice of talents, with less options; and 2) I personally liked the idea that my talent selection was a relatively permanent (although not absolutely unchangeable) commitment. Having a large selection of relatively fixed talents made my character feel unique, instead of a carbon copy of every other priest or rogue. "Oh, look, it's fight XXX, all priests make sure to grab talents Y & Z". Booooring.
The one thing I will say for the new talent stumps is that they do one thing quite well: convey the new Blizz ideal of "Less is More!"