Mage benefits:
--> lots of burst DPS
--> lots of AoE DPS
--> heavy crit (against non-resilient targets)
--> Good CC in PvE (against certain targets)
--> Trinket+PoM+Pyro
--> Good itemization from quests almost immediately in outlands
--> few abilities to worry about putting on hotbars
--> Blink for getting out of stun/root/etc.
Mage problems:
--> Mana use in longer fights or where lots of AoE is required
--> Squishy
--> No pets
--> Squishy
--> Did I mention they were squishy?

Shaman (elemental) benefits:
--> Decent single-target DPS
--> Limited AoE DPS (chain lightning and fire nova totem only)
--> Group heals
--> totems, Totems, TOTEMS!
--> Chain mail, so they can take a few hits while you recover
--> Fast single heals
--> totems, Totems, TOTEMS!
--> FROSTSHOOOOOOOOOOCK
--> Tremor/Grounding totems
Shaman (elemental) issues:
--> Limited AoE
--> totems limit your mobility
--> Mana use is high
--> DPS isn't as high as with pure damage classes
--> Itemization at lower levels sucks balls
--> zero worthwhile CC

Warlock benefits:
--> Pets = pocket tanks
--> Can DPS while moving and while facing away from the target
--> Can heal pets yourself
--> Can heal self with siphon life/drain life
--> lots of CC (fear, horror, AoE fear, seduce, silence)
--> Mana drain x4/x5 = empty casters
--> unlimited mana with a group healer (i.e.: priest)
--> Decent AoE with hellfire/rain of fire
--> Can stack a lot of abilities onto a few macros
Warlock issues:
--> Pets can't tank uber elites very well
--> no innate fear/CC escapes
--> Squishy
--> damage takes time to apply itself (18-24 seconds)
--> CC doesn't last as long as sheep
--> Best PvE spec different from best PVP spec (*caveat emptor*)

That's my short list. There's more but that should be enough to chew on for now.