People meme on Paladins, but let me hit you with some truth;
Their tanks are great for dungeon content. They have excellent AoE threat once they get Consecration coupled with Righteous Fury. They do, however, fall off hard in raids except for an off-tank role against certain bosses and trash mobs. The drawback is having to stop and drink, but if you can handle your mana bar (not always slamming max rank spells) and use JoW properly, this can be mitigated quite well. Mana pots from alchemy help.
They are the best single-target healers in the game. Once specc’d and geared, they can essentially have infinite mana and a rapid fire heal. Not much to say here.
Their DPS really isn’t trash-tier until later in the game, despite the memes. And even then, a Ret Paladin will do fine for most dungeons. Especially Dungeons filled with Undead. The thing you have to remember as a Ret Paladin is that you are a hybrid class, which means you shouldn’t be looking solely at that DPS meter. You should be buffing, Cleansing poison and disease, throwing out the odd heal, throwing out CC with Turn Undead or Hammer of Justice, throwing BoP on the healer or DPS who drew aggro or BoF on those snared / rooted…
Example: Ran Gnomeregan yesterday, had an under-levelled Paladin. He was sitting at 27 which is fine for the first part of the instance, but struggles the lower down you go. Firstly, the Paladin made sure to keep us all topped up on buffs. For the first part of the instance, his DPS was fairly respectable as well. But when we got down to Electrocutioner 6000 and onwards? His DPS fell off hard as he was barely hitting. So what did he do?
He transitioned to off-healer. And when we got to Sicco Thermaplugg? He spent his time destroying the walking bombs and shutting them down, while continuing to help the healer, who was also slightly under-levelled. A low-level Warrior, Rogue or Mage etc could not have provided that same versatility, range of buffs and what-not.
There’s also that sweet Resurrection spell, which people are always thankful for if the Healer dies. Or if your healer is a Druid and thus doesn’t wanna blow their single battle-rezz.