When the zones change in Cataclysm, the change will be permanent. Once the Barrens is split in half by Deathwing, there will be no way to see or access the original Barrens content. At least that's the way things were described at BlizzCon, but it's always possible this could change.
"AoE protection paladin, by far the easiest toon to level. Why kill 1 mob fast when I can kill 20 in the time it takes you to kill 3 one-on-one?"
If you know the class and have the gear, leveling a prot paladin isn't a hard thing. But chances are most people don't know the class and don't have the gear -- in which case retribution spec is your friend. Also, how many times do you really have to kill 20 mobs? In the old world sure, it happens enough, but you can't really get into the meaty prot paladin points at that point anyways. Most newer quests these days require you to kill one or two mobs in close proximity to each other, and then go over and find another few mobs to kill. You rarely can pull entire areas and be effective with your AoE to the point of having an advantage over being retribution.
What I recommend is every paladin carries both a prot and retribution spec during leveling. It will only increase your usefulness.
"Do you think that Blizzard will ever produce a looking for raid function that automatically organizes raids for pugs? Is it even possible?"
Blizzard has implemented this somewhat in game now via the "/lfr" command. It only works on your server, however, and doesn't automatically group you up -- someone still has to do the invites.
As far as cross-realm raids go and having the PUG form automatically, it's technically possible, since the difference between finding five people and finding 10 or 25 people is only a few bits in the computer. Is it advisable right now? Probably not. Random dungeon groups work because most bosses are tank and spank -- you don't have to do anything beyond stand there and execute the primary roles of your class (well, you have to move a bit here and there, but that's it).
Raids require a lot more work and can be substantially more complicated. Just because it's possible to farm Naxx or ToC in pugs right now doesn't mean the entire community can do it -- but the entire community can do random heroics (for the most part). There is also the problem of reforming the raids and finishing them at a later time. The in-game communication system doesn't allow for this level of cross-realm communication right now, which provides an inherent barrier to any real cross-realm raid solution. But all of these are just technical challenges that could, at least theoretically, be easily overcome.
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