What started as a talent that was a very basic 15% damage reduction from melee and increased base shadow damage, has been changed to 5 talents in one; reduction to both incoming melee and magic damage about wow powerleveling, anti-spell pushback, 30% threat reduction, and ability for DoTs to crit. Blizzard has cautiously and slowly modified the talent into a final product that is impossible to dislike.

What seems to be the most effective approach for Blizzard, is making sure that the fundamental talents (31,41 etc) are as beneficial as possible to both PVE and PVP players. Going from there about wow powerleveling, they have seemed to taken a liking to making most talents useful for both, but throwing in a couple of one-sided talents that really address specific relative problems to the side approached. For example, in terms of PVE Shadow is doing relatively decent, and is continually improving. Before 3.1, Shadow was hurting for PVP viability, but with additions such as the new Psychic Horror about wow powerleveling (which seems to have slipped under the radar as it is ridiculously good and borderline broken), Shadow Priests have the potential to go from crap, to quite good overnight about wow powerleveling.

It goes beyond just Priests as well. While there is still a lot of work to be done, many classes have seen talent trees that have never been viable become useful and borderline well-rounded (or gimmick at best). In example, you have Ret for Paladins, Fury for Warriors, Arcane for Mages, Balance for Druids, and Survival for Hunters. If you go back to around season 1/2, some of these trees were so far off from actual balance, it was thought that it would simply never happen. That is not to say that as some specs have risen, others have hit the floor hard, but even those talent trees still have potential if they received even one or two intelligent changes like Shadow recently has.