Paladins aren't in Horde in Vanilla, so that's out if you intend to play with Nost. Plus, it requires you to enjoy the thought of sucking several dicks 24/7.
That said, it's heavily dependent, so I'll give a quick summary of each class:
>Mage
+Respectable single-target damage, top tier AoE damage, arguably best PvE CC in the game, only class that can effectively AoE farm if specced right, access to all major cities with teleport spells, can jew players with conjured food/water and portals. Also in fairly high demand in endgame groups as of now.
-A LOT of downtime, expected to make a fuckton of food/water for raids, Arcane a shit until Naxx (apparently), Fire a shit in the first 2 tiers, basically made of glass.
>Warlock
+Good single-target/AoE damage, pets + Life Tap make for fairly good soloing, arguably OP PvP CC, niche CC, can summon players, Felhound is obscene in PvP, DoTs.
-Expect to devote at LEAST whole bag to Soul Shards, DoTs basically useless in raids due to DoT limit, Fear is pretty bad in PvE.
>Priest
+Easily best healer in the game (and healers are constantly needed), Shadow generally considered fairly broken in PvP
-Expected to heal in raid environments (and most non-raid environments), since Shadow's DoT-heavy damage is mostly neutered in raids, a lot of downtime, generally a huge bitch to level (including Shadow till Shadowform), squishy as fuck (and usually the first target in a PvP environment) as a healer, minimal, if any AoE.
>Druid
+Can basically fill any role at lower levels regardless of gear, and can fill any role at 60 based on spec, basically flag-carrying gods in PvP, only class that can resurrect dead players while in combat, niche CC, one of the easiest classes to level, 30% speed at level 21 for ferals and 40% speed boost for all specs at 30, can (relatively) safely AFK just about anywhere starting at level 20
-Expected to healslut in raids,Post too long. Click here to view the full text.