There's an interesting editorial up at Kotaku Australia discussing the problems with hybrid classes, such as the Shaman and Paladin, and why ultimately they don't work:
People aren't looking for a class that can't excel at anything. Sure, players of hybrids will say they're fine being a lesser substitute, but when push comes to shove, no one wants to be left behind because a pure class does better.
The writer also says he'll be doing a follow-up, suggesting a replacement for the "Holy Trinity".