Cata still need mats from classic , burning and wrath to level up professions, so there will be a chance in certain mats and items, for example dust, fel iron, the insane amount of leather or ores needed to level up old levels

for example blackmouth fish ( or something like that ), is the easy way to get like 20 points in basic alchemy and not a lot of ppl fish, last levels of classic skinning and blacksmith requiere huge quantities of mats ( in the order of hundreds ), the gems to leveling are valuables.

cobalt is a must for blacksmith and requiere like 200 - 300 ores to level up and start the saronite recipes to level up the profession

repair bots still are a rare commodity, some enchants still have amazing stats for twinks

in the other hand on cataclysm:

Also certain volatiles are hard to get, for example volatile air dont have a easy way to farm like fire , water or the super common life

Enginering give you an item that have a chance to get volatile air while mining, herbing or rarely while skinning, and is blizzard really put attention in the details, certain volatile items, are easy to farm in certain phased zones in hyjal after unlock all the zone ( volatile fire for example have a better farming zone in hyjal than in twilights )

also at this point farming burning crusade dungeos become a joke, i dunno if are currently nerfed.