Tanking, gear and class scaling & class utility.

Quote from Blizzard staff

Tanking in WotLK
The goal is roughly equivalent. The goal is not identical. Identical would mean making all abilities identical, down to things like Divine Shield and Innervate. I also want to caution you all that there are plenty of players who are concerned about the degree of homogenization we’ve done to get the tank classes as close as they are. It’s cool for everyone to have their different visions for what the game should be, just make sure not to impose your vision on everyone else. There are eleven million people playing WoW….

As far as the other arguments are concerned…

-- You can argue that our design is for warriors to be the best tanks. But I can debunk that right here. It's not. They will probably be superior in some cases and inferior in others, but never to the extent that you bench the warrior and bring in another class for a certain fight.
-- You can argue that some classes need to tank worse than others to compensate for their other abilities or degree of hybrid-ness. But that is not our design.
-- You can argue that some classes end up being better tanks for some reason or another. This is a totally legit concern. I don’t think we have any evidence that this is really happening yet, but it is something we pay a lot of attention to.

-- In Classic WoW there was really only one tank, the warrior.
-- In Burning Crusade, we added two other tanks, but the design was that they were typically used in an OT role. There were exceptions of course.
-- The design for Lich King is four tanking classes. You should be able to raid in Lich King without a warrior tank, or without a warrior period. You can replace “warrior” in that sentence with 9 other classes. One of the reasons we spent so much time on the warrior Prot tree is we wanted to compensate them for removing them from the “best tank” pedestal, or at least making room up there for druids, paladins and DKs as well. (Source)

Gear and class scaling
Our general philosophy is that we don't want relationships to be too predictable. Much of the fun of improving your character is seeing the effects that different kinds of gear have on different rotations. To use an extreme case, if every piece of armor just said "improves stats by 1%" getting loot would be a lot less fun.

Similarly, we don't want classes to be too similar. It's interesting and fun when you switch classes from say a warrior to a rogue and see how different stats affect you differently.

Scaling is very important to us in the sense that we don't want certain classes or specs to completely fall down at certain gear levels. This has happened in the past. But what those levels are is very important. If mages dramatically overtake warlocks when everyone has 20,000 spellpower, then I just couldn't care less except possibly as an item of trivia. If and when WoW ever reaches numbers of that level, the class mechanics will have changed so much in those years that its just not worth worrying about at this time. (Source)

Class utility
On the issue of utility, yes our design is that nobody has an ability so awesome or critical that they are gauranteed a raid spot. In truth you can get all of the important buff and debuffs with a relatively small number of players (especially in a 25 raid) so you should be able to bring who you want after that. If some classes bring versions of a buff that are obviously and significantly superior to another's then that is the sort of thing we'll get fixed. (Source)