Paladins get a playstyle overhaul in the form of the new holy power mechanic as well as a return to an older seal system. Current paladin players might take a short while to accustom themselves to the new playstyle, but players rolling new paladins should get into the groove fairly quickly. There currently is no UI for holy power, but it would be a practical addition if developers fuse it with the interface similar to the new warlock soul shard mechanic or death knight runes. At low levels, paladins finally have an instant attack spell as Crusader Strike goes baseline but they also have three very distinct feels, thanks to the new specialization abilities. Here's what to expect for paladins in the first two battleground brackets:
Holy - Holy paladins get Holy Shock at level 10, along with passive mana regen, better healing, and pushback protection. Interestingly, holy paladins get a potent ranged offensive spell at low levels, a complete turnaround from the old paradigm where paladins didn't even have a proper ranged pull. There isn't much excitement in the first two tiers of the holy tree, but several talents help with crits and haste.
Protection - Tankadins get a full complement of PvP-friendly spells from the get go, from the silencing Avenger's Shield to more stamina to decent mana recovery. More importantly, protection paladins get access to the shorter 40-second cooldown of Hammer of Justice and longer duration Hand of Freedom with Guardian's Favor. PvP retribution paladins will pick those talents up at higher levels, but for now, they're part of a PvP tankadin's arsenal. The flip side is that they won't be doing much damage with a one-hander as shield-based attacks come much later.
Retribution - For now, retribution paladins get Templar's Verdict, a new spell that works off holy power. It's pretty much Crusader Strike on steroids, and it's nothing fancy. It also delivers a pretty clear message: retribution is all about dishing damage. There's little by way of utility at early levels, so the retribution toolbox is far from complete and rendering them somewhat one-dimensional in PvP. There's the crucial Pursuit of Justice on tier two which should help their mobility a little.
Discipline - As with other specs, discipline priests get something distinctive to help them level up. In this case, it's the laser beam version of Holy Shock, Penance. More interesting are the talents on tier two, such as Evangelism and Archangel, which will define discipline priest play as they utilize Smite to maintain stacks of evangelism to consume for Archangel. Also, early on, discipline priests will have access to faster Mana Burns (available at level 24 on live, but level 58 in the beta -- not sure what will stick come Cataclysm).
Holy - The interesting thing about the holy spec is that they get Desperate Prayer, an instant, big self-heal on a long cooldown. It's down from it's current 2 minute cooldown, but at 45 seconds, it's not a terribly great PvP skill. It still certainly goes a long way towards survivability in the lower brackets. Much later on tier four, players can pick up Binding Prayers to make it some form of intelligent Binding Heal. The talents on the first two tiers pale in comparison to discipline, as well, providing no new game mechanic to play with. Even Inspiration will hardly be palpable at lower levels, so don't expect a lot of holy priests in lower brackets.
Shadow - As with Moonkin Form, Shadowform becomes available at tier three, well out of reach of other specs but early enough to be of use to leveling players. (EDIT: Reader Nirva points out that talent points are distributed at odd levels after the 10th, so Shadowform should be available at level 29) This puts the talent slightly out of the 20-29 bracket. This means players won't be able to get the most out of Improved Mind Blast. That said, low level players can make good use of Mind Flay, which is the bonus ability that comes at level 10.
One big game changer for rogues is Recuperate, a baseline ability that allows rogues to heal using combo points, learned at level 12 in the current beta. This means rogues will have ways to recover health while in combat, which is huge in the early game where high stamina doesn't quite come into play just yet.
Assassination - On the surface, rogues don't get much differentiation at lower levels. That is, until you see an assassination rogue unleash Mutilate, their bonus ability. This is bundled with better poisons and deadlier daggers. This should come in handy in recognizing assassination rogues, who are likely to also have Deadly Momentum, which importantly refreshes Recuperate. There's also the extremely PvP-friendly Quickening which increases movement speed and Recuperate healing. Expect to see Sap used very often as Blackjack comes into play in conjunction with their openers.
Combat - While combat appears to remain a sub-standard PvP spec considering that the bonus ability Blade Flurry isn't a particularly stellar ability on the battlefield, several talents on the first two tiers should make it interesting to play. The first is Improved Recuperate, which should give combat rogues an added layer of survivability, another is Improved Sprint which gives them an enviable counter to snares, and then there's Improved Kick which gives them a blanket silence. Other specs will have access to these PvP talents at later levels, but in the lower brackets, only the few combat rogues will be able to use them.
Subtlety - The whole distribution of abilities at low levels is guaranteed to make the game so much fun again. Imagine rogues Shadowstepping all over in the lower brackets -- it's just visually cool. Unfortunately, Elusiveness is only about a third of its usefulness at these lower brackets but Waylay is an intriguing PvP ability if the rogue manages to regularly use it to cripple opponents.
Like paladins, shaman get a baseline weapon strike in Primal Strike, which should come in handy between spell cooldowns. They also get more distinct feels as with the other hybrid classes.
Elemental - Two words: knockback effect. How fun is it when low level players can knock enemies out of cliffs? Sure, it can only happen in the 20-29 bracket of Arathi Basin, but you've got to admit getting Thunderstorm at level 10 is extremely cool. Aside from that, though, there's nothing too exciting for elemental shamans at lower levels, just a whole lot of crit, more damage, and a sweeter lower cooldown Fire Nova. Doesn't matter, though ... they get knockback.
Enhancement - Enhancement shaman will make the most use of Primal Strike, along with their bonus ability Lava Lash. They won't have as much in their offensive arsenal at low levels, with key abilities coming much later, but they do have one very important talent for PvP -- instant Ghost Wolf and faster movement. Everything else is gravy.
Restoration - What else would a restoration shaman get but ... Earth Shield? While obviously not an offensive force, most of the talents in the first two tiers of the restoration tree right now contribute nothing but cheaper heals and mana regen, which theoretically means that restoration shaman can keep themselves (or their friends) up for a very, very long time. Not much of a change from endgame in live, right? Well, at least you get a taste of it early on when Cataclysm rolls around.