It seems that most classic reputation rewards are no more interesting because they're low level recipes and items. WLK rep reward gears are also not attractive even for leveling alts because Cataclysm gears for the same character level have much better stats.
However, I just learned not all pre-MoP reputation are irrelevant in MoP, like Sha'tar reputation is the only way to get discount for guild items, although it's not gear reward but it can save me a lot of money on guild heirlooms. Also it's a good idea to get to exalted with Orgrimmar/Stormwind instead of other main cities for the discount for master riding.
So the question is as of MoP 5.3, which factions' reputations should I prioritize that reward valuable end game gears, recipes, special discount or unique items, given I have only limited time to play the game?
(I am not collecting mounts or after any grinding achievements like '60 Exalted Reputations'.)
Edit: If asking for a complete list is too much, can I have a list of the reputations not worth pursuing? E.g. Thunder Bluff because the rewards are only low level gears and mounts.