DPS warrior is probably the most difficult class to get into a raiding guild.  Almost every guild has some already, and those that they do have are expected to be able to tank when the time comes.

The most saught after class/spec is imo resto druid.  Holy Paladins are still somewhat hot commodities on horde, I imagine resto shamans are similarly desired on alliance.

As far as dps classes go, most raiding guilds like warlocks for their ability to sustain dps even while they are moving and banishing demons and elementals, and a meleer who is good at interrupting spell casting will be appreciated.  Other casters adore shadow priests for their mana restoring abilities and debuffs.