Class specs on demand / most needed

Edit: K so, for organized guilds do w/e as you're likely going to have lots of people willing to fill up the desired "required" spots as some people view them(key raid buff providers).

For dps it looks like boomkin/ele sham/spriest are what people struggle to find in pug groups. In my experience shamans are always in demand resto for heals and ele for caster support as a ele + spriest combo cover the boomkins buffs w/ the bonus of bloodlust/heroism. Other than those 3 people always ask for carry dps, which entirely depend on the tier we're in. Atm dks, warlocks, boomkins are insane if they're played well. Rogues are very good too though I'm biased against them due to the extremely limited raid utility they provide, hunters are good, rets are good if played well though for every ret that's -1 healer/tank. As tiers progress mages will outshine warlocks eventually in terms of raw damage potential, warriors will come online as they get ArP and will be absolute monsters later on, oh and boomkins stay strong as long as they can maintain their crit threshold(boomkin w/o it, is like half a class as they're entirely dependent on Eclipse).
Hmm, for enhance I've only seen 1 good enhance shaman on the server. Literally only 1 going beyond 6k dps, single target. Likely because frost dks provide similar buffs and are far easier to pull off that much dps atm. Note on hunters: MM and survival are equal imo, as there isn't enough ArP available to push MM ahead of survival.

Tanks: They're about equal i'd say. The hard part is finding good tanks that know how to use their classes abilities properly. Multi add fights where you need to block a lot of swarm damage? Warriors are king w/ 30sec downtime of shield block, not to mention the 3 other cd's they have. Huge health pool for Faerlina? Bears are the great for this. Paladins are obviously OP with their Ardent Defender and easily obtainable high avoidance values against few targets(holy shield falls off too fast against swarm mechanics too really benefit from block tanking in that situation). Dk's have the highest single target threat period as long as they're specced right, ***** to heal though early on but as you gear them up they become more sturdy.

Healers: Everyone and their mother needs shamans that can heal and knows how to use bloodlust when it's demanded of them. Nothing more annoying than asking your shamans for bloodlust like 8 times in a row because they're tunnel visioning(Especially when we want to maximize the BL buff during dps windows). Most in demand healer by pugs is paladins on horde side from what I've seen. Disc priests are surprisingly common, though are always welcome especially if you have pain suppression put into a mouseover macro or is set up with an addon to do the same. Resto druids are extremely powerful in these early tiers as incoming damage isn't a snipe fest yet. Holy priests... their niche is below 50% hp healing and currently in naxx I haven't found that to be an issue as of yet as long as everyone wears frost resist gear on sapph, etc. Though in later tiers they'll start to shine more and top raiding guilds will likely transition a resto druid or 2nd resto sham into a holy priest eventually for their 25m comps.