If you plan to twink on 70, I highly recommend you to get separate tbc account for that purpose. You can always make a twink with a friend, get RAF for triple xp for levels 1-60 and the leveling will go really fast even without heirlooms. Sure it's another 15$ per month but if you aren't playing on your wotlk account, you can always de-activate that to save the money (though it's pretty nice to be able to earn some money / boost on your 80lvl characters).
61-70lvl AV in our battlegroup (rampage) suddenly died couple of weeks ago, so we aren't getting queues unless it's AV weekend. That might be the case in your battlegroup as well, so prepare to play against 79lvls in other battlegrounds (unless that will be adjusted in cataclysm). I have retri paladin and warlock myself and from my experience, retri paladins are one of the best in these brackets. I have a shot to beat anyone in 1v1 (against other twinks it's always 50:50 though) and it's not unusual to beat 2-4 higher level opponents at one time with succesfull cc / cd / los abuse. Paladins are one man army, you have couple of powerful cooldowns, decent cc, healing spells, dispel, burst and aoe. I'm afraid that ret is going to be less effective in battlegrounds in cataclysm but haven't tested myself so can't really say for sure.
I do pretty well with my lock as demonology as well. With metamorphosis up, I've beaten up to 5 people same time. I have 16k hp and 500 resi and with soul link and other talents, I'm very durable. Affli feels pretty weak right now since you have to choose between haunt and soul link. Destro has nice burst but missing one spell can cause you to die and they are also very fragile without soul link. In cata demonology seems to be very powerful and also soul link will become trainable.
Overall, also warriors seem to be good right now but they pretty much require a healer to be around you, unless you are prot with good gear. Hunters have nice burst with good gear but high level casters will wreck you up unless you can kill them quick. I haven't seen many rogues around but right now they are very squishy and being lower level = people will easily get you out of stealth. Rogues are going to get some nice buffs in cata though.
Mages and priests are good in 61-70 but haven't seen many playing in 70-79. Spell casters have high penalty to hit against higher levels and missing an important spell can easily get you killed. I occasionally miss hammers or repentances on my paladin too, but that haven't been a problem really. Shamans will get some nice buffs for all specs in cata, but at least elemental is really having problems with missing spells. Of all casters, demonology warlocks are suffering least from hit penalty from my experience.
That's the current situation from my point of view. If you got interested in some specific class, I'd suggest that you research more about that class (what they will get / lose in cataclysm). I can't really say which class is going to dominate in cata since I'm not playing in beta or ptr so it's all speculation until it comes.
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