Depending on class really, but concerning PvE, I would say Troll or Orc you can almost not go wrong with concerning dps. Tauren for tanking, although the health buff is pretty minimal since the change in TBC... Orc is not a bad idea for tanking because of the stun reduction. Undead not too bad for the CC break. For healing... Troll/Orc/Belf for paladins arcane torrent for some minimal mana gains, trolls for haste, and orcs for the spell power buff. There are a lot of options to be honest. And of course the pandas have the wonderful food buff, which is turning out to be pretty legit.
DPS troll, orcs, goblin (haste, attack power, haste)
RDPS troll, orcs, goblin (haste, and spell power buff, haste buff)
Healer troll, orcs, BE (Haste, spell power, and mana torrent -to get mana-)
Tanks Taurens, undead, orcs (Stamina buff, Work for some CC and tough of the grave help a little to survive a little more, stun reduction)
The goblins are the more practical race... why? they have a movil bank, everything is cheaper for them, have the rocket to do a little more dps, and have the rocket to run away like a little girl XD
I'm just wondering and have a Question: Since when is Orc Bloodfury 2257 Speelpower ((2257/120)*15) = 282,125 on Average or Goblin 425 Haste or Troll Racial (((425*20)/180)*10) better than 300 int that the pandaren Bonus flat provides. I know that haste is multiplicative and not additive but hey.. I dont get it.
I race changed from 7 years as undead to panda, apart from the general awesomeness that is having a character that bounces when it walkes, the 300 intell I get when in raid is awesome (imo).
And it's good for any spec, moreover.