Though I was initially in favour of the new "all classes perform all roles equally" philosophy (e.g. all tanks will perform at the same level, all dpsers will perform at the same level etc), I'm rapidly getting disillusioned with it. It seems like there's absolutely no point in playing anything other than a druid, since they can tank, heal, melee and spellcast as good as all the pure classes. Fair enough they may have a little bit less flavour or utility in their dps roles. Fair enough they will need to gear up for each role as well, but if I want to tank or heal on my Mage I can't do it. I need to reroll, level and gear up a whole other class.
I'll need to see how it all pans out, but I'm guessing the hybrids will be within a whisker of pure classes in terms of dps (and exactly equal for tanking and healing), but with gimmicks added that make use of specific abilities of the pure classes (such as Warlock tanking, Mages spellstealing). Unfortunately, I can say from experience that being brought along to cover for gimmicks is no fun at all.
For god's sake, if you chose a Shaman that says "uses totems and buffs groups" why on earth would you complain that you're not doing equal damage to the Mage which was originally described as being weak but with the highest ranged damage? You got exactly what you picked.