there have been several posts about this lately. i think the consensus for the only reason to do a lvl5 twink would be the fishing hat. in which case go for it! here is a list of the additional abilities each class gains over lvl1
Shamans: Stoneskin totem & Earthshock
Paladins: Blessing of Light & Judgement of Light
Warriors: Charge & Rend
Druids: Rejuvination & Moonfire
Priest: Shadow Word: Pain
Hunter: Aspect of the Monkey & Serpent Sting
Rogue: Backstab & Pickpocket
Warlock: Corruption & Curse of Weakness
Mage: Conjure Water & Frostbolt
i put them in the order of how i'd play em ... but it's purely subjective