There's a new philosophy in Wrath (actually, already here in the post-3.0.2 world) when it comes to who you should bring on your raids. In short, as the headline says: pick the players you like, not the classes and specs you need. In all of WoW until this point, there have been certain specs that were virtually required in various parts of the various raids. Which specs these were has changed with time, but think of Shadow Priests' mana battery capabilities, or stacking Shamans to get many Heroisms/Bloodlusts. To be a cutting-edge raid it was simply required to prioritize "correct" raid composition over other factors. Yes, your players needed to know how to play their class, but beyond that, you took what was needed for the fights you were doing. And you took a Warrior main tank, generally speaking.
All that is set to change forever in just nine days. Now the classes are much more interchangeable with each other than before. Some prominent buffs have been nerfed, and in one notable case, distributed: Shadow Priests, Survival Hunters, and Retribution Paladins all give mana back with Replenishment. In general there is more than one class that can provide most types of buffs and debuff. Also, many buffs that used to be group-wide are now raid-wide, such as Paladin auras and Shaman totems. This makes it much more likely that picking two tanks, three healers, and five DPSers out of a grab bag will get you decent coverage on buffs and debuffs.
Another place where this new philosophy shows through is in hybrid DPSers. The old philosophy was that hybrids may be weak in DPS, but they were strong in utility. Now specs such as enhancement Shamans are expected to be doing very close to the DPS of "pure" DPS classes such as Rogues. Blizzard posters maintain that differences in player skill and gear are likely to be greater than inherent differences here, so that in order to get the highest DPS, you'd take a shaman over a rogue if you knew the shaman was a better player. However, pure DPSers are meant to be doing slightly better than hybrids if all other factors (gear, skill) are held constant and the fight mechanics do not favor one or the other.
Finally, every tanking class can main tank now. Previously, if you were doing a progression raid, you were best served bringing a Prot warrior, partly for their large array of contingency buttons, partly due to mechanics that heavily favored Warriors, and partly just because it was tradition. We can't do much about that last factor, but the first two are largely gone in Wrath. The other two previous tanking classes (Paladin and Druid) have been given some nice new tricks, and Blizzard has pledged their support in making all four tank classes (remember, Death Knights are coming) equally viable main tanks, which should mean no special fight mechanics that require specific classes.
Some players feel threatened, worrying that they don't have any unique spells any more that guarantee them a raid spot. That's the entire point: no spec is guaranteed a raid spot any more. Earn your spot by being a good player, by knowing your class and the fights, and by coming prepared.
Personally, I find this a very exciting trend. That might just be because I mostly play hybrid classes, but honestly, six out of ten classes are not pure DPS classes, which at least to some extent qualifies them as hybrids. I know we have more options than the pure classes -- we can respec to heals or tanking if we want to -- but it's untenable to keep us low DPS simply for that reason. And if they can achieve their stated goal of making it easier to form a successful raid just by bringing people you like to raid with (in the right proportion of tanks, healers, and DPS), that would be excellent in my book. After all, this game is about fun, and I never have more fun than when I'm playing with friends. We shouldn't have to choose between fun and success.