A paladin's core strength lies in her ability to outlast an opponent. This is what made retribution so fearsome in addition to its formerly extremely powerful burst damage. Paladins wear plate armor, can heal, self-dispel, and have a number of damage mitigation and immunity abilities that enable them to survive assault from most classes. Paladins rely on self buffs, and they have two kinds: blessings and hands. Blessings are basic stat buffs such as Blessing of Might, Blessing of Wisdom, and Blessing of Kings. In PvP, expect to see paladins with Blessing of Kings as it also provides a boost to health. Hands are situational buffs that have cooldowns and you should expect to see paladins use Hand of Freedom frequently. This will be key to a lot of strategies, so watch out for it. Paladins can also use Hand of Protection, although rarely on themselves because it causes Forbearance, which will prevent them from using the more important clutch spell, Divine Shield.
Let's run through a paladin's commonly used abilities in combat:
This level 80 spell is key to paladin PvP. Paladins will have it up at almost all times, whether on themselves or a friendly target. If you see a retribution paladin without this buff up, rest easy in the knowledge that you're probably up against a beginner (or a thankfully forgetful opponent). If you have offensive dispel capabilities such as Purge or Dispel Magic, make sure to spam them against paladins until they're completely stripped of buffs. Sacred Shield scales, so it can mitigate a lot of damage and prolong the fight unnecessarily.
Notice how it's another 'shield' spell? This should get the point across that paladins are about defensiveness. Divine Shield is also known as the vaunted paladin 'bubble', and should be visible as a 'bubble' of light around the paladin. When this is active, the paladin cannot be harmed in any way and many paladins use it as a last resort. If you play a priest, you can use Mass Dispel, and if you're a warrior, you can use Shattering Throw, an ability Blizzard made specifically to counter Divine Shield (it even has the same 5 minute cooldown).
Always watch out for this ability. While it is the paladin's most powerful, signature defensive spell, it also indicates their greatest weakness -- a period of Forbearance. In Arenas, a paladin who has used up her Divine Shield becomes a prime target for a switch. The same applies for Battlegrounds and world PvP. Once the ability has been used or if a paladin has the Forbearance debuff, they are at their weakest. Unleash as much damage as you can during this period.
A paladin lasts only as long her mana. As long as a paladin has mana, they can be dangerous opponents. Divine Plea is an important spell that paladins use to quickly regain mana, and the smart ones will use it long before they go OOM, usually almost every cooldown. The good news is that it's dispellable. If you have any sort of dispel ability, remove Divine Plea, even on a holy paladin. The healing penalty is something they would gladly incur in order to regain mana, so don't let them have it.
Arguably having one of the sexiest graphics in the game, appearing as luminous wings of vengeful light on the paladin, Avenging Wrath is another cooldown ability used extensively in PvP. Paladins use this with abandon more than other abilities, particularly retribution paladins, who have it on a relatively shorter 2 minute cooldown. As fearsome as the ability is, it suffers one fatal flaw -- it's completely dispellable and, if you're a mage, Spellstealable. It also triggers an internal cooldown for abilities such as Divine Shield and Hand of Protection, which means that while a Paladin is most powerful with Avenging Wrath up, they are also ironically the most vulnerable. Smart paladins will be extremely wary of using this ability against shamans, priests, or even warlocks with a Felhunter out or a warrior with a shield. Instead of looking at this ability with fear, think of it as an opening. For thirty seconds, the paladin will lose access to his defensive fallbacks, making it a great opportunity to strike.
Hand of Freedom
This single spell makes it appear that paladins have great mobility, but players who understand that this is a buff that lasts for only 6 to 10 seconds will know that paladins are supremely kiteable. Removing Hand of Freedom makes paladins susceptible to physical snares or Curse of Exhaustion. It's on a 25 second cooldown, so time snares to land immediately after Hand of Freedom has expired and it will be easy enough to create some distance between yourself and the paladin or, if you're a melee class, keep them close.
That said, in addition to Hand of Freedom, paladins are able to Cleanse magical snares, making them that much more difficult to kite. Cleanse is also frequently used to keep paladins free of DoTs and other debuffs, which adds to their survivability. It can remove magic, poison, and disease effects and is relatively cheap to cast. This is an underrated paladin spell and many fresh players and unskilled PvP players will forget to cast this often. You should get an idea of a paladin's skill level by applying some debuffs and seeing how she reacts. This is one of the most powerful tools in their arsenal and a paladin who uses it often and well is a force to be reckoned with.
Hammer of Justice
So paladins can protect themselves with all manner of shields and self-dispels, and they can stun you, too? Ouch. Aside from Divine Shield, paladins are feared and loathed for their stun. It's on a one minute cooldown, although if you've PvP'd enough times, you would probably swear they had it off cooldown all the time. Talents in the protection tree can bring the cooldown down to 30 seconds, but most retribution pallies will have it at 40 seconds. The good news? You can safely use a cc-break against this because paladins don't have anything else. Retribution will have Repentance, but that breaks on damage. A paladin who uses Hammer of Justice on a target wants control of the fight (or counter a spell). Don't let them have it. Break the stun and waste the paladin's cooldown.
Hammer of Wrath
Finally, you should be reminded that even the most harmless-looking paladin has access to a few offensive spells, most notably the Hammer of Wrath, which can be hurled on targets at 20% health or less. Granted, being that low in the battlegrounds probably mean you're due for an appointment with the Spirit Healer, anyway, but always mind your health around a paladin. Two things to remember: Hammer of Wrath has a travel time and it fizzles if the target becomes ineligible. This means that even if Hammer of Wrath is an instant cast spell, there's enough time to pump up your health above the 20% threshold to cancel it. You can use AddOns such as MikScrollingBattleText to warn you exactly when you have or are about to cross that threshold.