A note on paladin spellcasting

In addition to their considerable abilities to survive, don't forget that paladins are also a healing class. This means that any paladin regardless of spec is capable of healing themselves, albeit with varying degrees of efficiency. Fortunately, they have two basic heals that are easy to remember: Holy Light and Flash of Light. Protection and retribution paladins will hardly bother with Holy Light, but it is the absolute worst PvP offense to allow a holy paladin to get off a Rank 13 Holy Light. It is a 2.5 second cast spell that can change the complexion of a battle by bringing a paladin or her comrade from the brink of death to very nearly full health. One important thing to remember is unlike other spellcasters, paladins have only one school of magic -- Holy. Interrupting, or even better, Counterspelling a paladin in the middle of casting a spell will lock them out of their only school and consequently lock them out of most of their abilities.

Holy

While retribution paladins will be plentiful, the upper echelons of Arena PvP are occupied by holy paladins. As mentioned, a paladin allowed to cast Holy Lights unmolested is a formidable force. It is far better to allow a paladin to escape with a few Flashes of Light rather than a full-on, high-crit Holy Light. Always have your enemy cast bar active so as to know what spell they're casting and watch out for Holy Lights. As with all healers, Mortal Strike effects work well.

Of course, it would be great folly to think that holy paladins are limited to Holy Lights. The vastly improved Holy Shock is a powerful tool that can be used both offensively and defensively. It's on a low 6 second cooldown, so holy paladins who plan to go on the offensive can easily do so, pairing the ability with Shield of Righteousness to inflict some damage on opponents. Even though holy paladins do not deal a lot of damage, they have the ability to keep themselves up indefinitely, allowing them to whittle away at opponents who can't heal themselves. Never underestimate a holy paladin's ability to shift to the offensive, so always watch your health.

It is usually wisest to fight a holy paladin up close. They have poor mobility and must periodically stop to cast a heal. They will run, stop to heal, and run again. Limiting their mobility will bother them greatly, even if their heals aren't disrupted directly. Aside from Hand of Freedom, they are unable to remove physical snares such as Hamstring and Wing Clip, so bother them with that if you must. Even dispellable snares and roots such as rogue Crippling Poison will have them casting Cleanse instead of heals, which should help you in the fight.

Stacking debuffs on them will distract them from healing, and attacking them (physically) will bother their casting. Many Holy Paladins will fire off a Holy Light just to proc Light's Grace at the start of a battle. This is a magical buff and should be dispelled if possible. Offensive dispels will work wonders against any paladin. Always remove their buffs, and be mindful of where they apply Beacon of Light. In Battlegrounds, they usually place it on themselves, making it easier to spot and remove.

Because holy paladins rely on spellcasting more than the other specs, who can overcome a magic school lockout through melee attacks, they suffer from silences and interrupts more but are able to recover relatively quickly. Force them to blow their cooldowns by stunning, silencing, and interrupting them. When faced with a stun-lock or silence + high burst damage, many holy paladins will instinctively cast Hand of Protection or Divine Shield. Essentially, a paladin who has blown her cooldown will be an easier target, but expect a combat reset -- meaning expect the paladin to get up to full health one or more times during the fight. Watch out for activated abilities such as Aura Mastery, which will basically allow the paladin to obscenely spam Holy Lights for six seconds. Let them have that brief moment of glory.

As mentioned, a paladin's mana is a paladin's life. Whittle their mana down to zero and you've won the fight. Easier said than done, considering paladins have Divine Plea and holy paladins can be very mana efficient when they crit -- and holy paladins crit quite a bit. While draining mana isn't as effective as it once was, it's still a good tactic to use with the proper timing.

Capitalize on their poor mobility, long casting times -- Curse of Tongues works exceptionally well against them, and inability to remove enemy buffs. Holy paladins are a tough fight unless you play a class with specific tools to counter casting and reduce the efficacy of heals.

Protection

As difficult as holy paladins are to kill, protection paladins in PvP are even worse. A ridiculously high health pool, powerful damage output, and considerable tools for longevity make protection paladins awful fights for many players. Protection paladins have a good number of mitigation talents, ridiculous mana regeneration with Blessing of Sanctuary and Guarded by the Light, and dishes out pain with properly glyphed Avenger's Shields among other attacks. This makes protection paladins absolute hell to fight with a melee class. If you are a rogue, for example, my best advice would be to vanish. No, really. Get some friends. Protection paladins are too much trouble to solo and is arguably the worst kind of paladin to face off against.

Despite the nerfs, protection remains a formidable PvP spec with enough mitigation and a high health pool to shrug off most attacks and enough healing ability to recover should they get low. Even interrupts won't faze them too much because many of their instant abilities will work even with a school lockout and a few seconds of silence won't be enough to cut them down, either. Protection can also stun enemies every thirty seconds thanks to talents. Up close, protection can deal massive damage using Shield of Righteousness and Hammer of the Righteous, as well as keep up Holy Shield for even more mitigation and retributive damage. Generally, melee range is bad news -- even their Judgements slow melee attacks -- and you have their work cut out for you against protection.

The small upside is that casters have a much easier time against them. Keep them on the defensive by kiting them, even using magical snares and roots. Be sure to dispel Hand of Freedom or be sure to maintain your distance throughout the duration of the spell, as paladins have no rush or charge ability that can close the distance. The basic spell you need to watch out for is Avenger's Shield, which is an interrupt. It has travel time and a barely noticeable delay before the silence effect, but it can easily ruin a caster's plans. Keep track of Avenger's Shield's 30 second cooldown using AddOns like Afflicted 3.

Paladin mobility, as pointed out, is very low, and players able to keep a protection paladin at bay will eventually win out. Keep at max range at all times -- preferably at the 40 yard distance with proper talents -- and you should be safe even from Avenger's Shield. A protection paladin not hitting anything and not capitalizing on Blessing of Sanctuary won't last very long, so pour on as much damage as you can from afar. Drain their mana as much as possible, because they'll have limited ways to regain them. As long as you dispel Divine Plea as soon as it goes up, you should be alright. I forget that Divine Plea cannot be dispelled on a protection-specced paladin because of Guarded by the Light. However, you can manage to prevent a 100% uptime. (thanks to Adjatha for pointing this out!)

If you play an affliction warlock with the talent, use Curse of Exhaustion at will. If they use Hand of Freedom, either eat it with a Felhunter's Devour Magic or Fear the paladin for the duration of the buff. The idea for casters is to keep them at a distance at all times. If you're a caster squaring off against a protection paladin, have fun and good luck. If you're a melee class, it might be best to find another target.