Hunter is the easiest to level. They are good in PvE and good in PvP also.
Most enjoyable class is the rogue, but they are TRULY a pain to level. They are also rather easy to play. They are good in PvE and good in PvP
Warlocks have access to a pet. They are somewhat easy to level, but you'll have those "FUCKING HELL!" moments with them whilst leveling. They are quite good in PvP and quite good in PvE
If you go tanking has a paladin leveling will be slow until you reach 60. Not a class if you are in a rush to 80. At the moment most people consider the retribution spec to be overpowered in PvP, which is kind of true. They are considered the best PvP healers giving their survability if they use the protection talent tree combined with healing gear. They are good in PvE and good in PvE.
Death Knight's are somewhat bad in PvP at the moment. I wouldn't suggest leveling one and having high hopes for PvP. In PvE we have only 2 remaining viable damage dealing talent trees. The 3rd one was over nerfed and it's going to be a bit until it will become viable again. PvP not so good, PvE in between decent and good.
Druids are the "jack of all trades". They are good healers, they are good damage dealers and they are good tanks. They can transform into animals. The basic melee damage dealing form is the cat. Its abilities are basically a copy of the rogues' abilities, only with different names. The tanking form is the bear form (Obviously), having abilities like that of a warrior, only with different names. Healing form is a tree. PvP good. PvE good.
Warriors scale horribly with gear. A fully blue geared warrior will not be able to do has much damage has a fully blue geared Death Knight for e.g. The better the gear, however, the better the damage. Well geared warriors have the ability to out-damage any class. In PvP they are extremely good...with good gear. PvE good, PvP good, but again requires very good gear.
Mages can do quite good damage, but they have only one spec with which they can ditch out damage in PvE, which is the "Fire" Talent tree. The Arcane talent tree is "trash" damage, meaning good for the first minute, crap for the rest and the Frost Talent tree is utterly dedicated to PvP. The frost talent tree is also famous for being the bane of all melee classes. PvE good, PvP good.
Shamans are again just plain good. PvE wise they can ditch out quite a lot of damage, they have the choice of going toe to toe with an enemy or casting from afar. They also have a healing spec, but you'll be finding yourself spamming one spell, occasionally 2. PvE good, PvP good.
Really nothing what to say about priests. They're awesome healers, not so good damage dealers. PvE awesome, PvP awesome.
Did I just spend 5 minutes writing all this? Holy crap I'm hopeless...