As with all things in WoW, it depends on what you want out of the class (Locks in this instance).
For instance lets look at the three classes' racials as they apply to the Warlock class:
Quote:
Orc:
Main contributing racial: +5% Pet melee dmg.
Assuming your going Demo lock (With your demons being the focus) 5% increase in damage is a nice little incentive.
Quote:
Undead:
Main contributing racial(s): WotF, Cannibalize.
IMO if you want total synergy with the Affliction build, you'll want an Undead character.
Cannibalism works so well along with Lifetap (and combat healing in general) that you'll rarely sitdown for a drink, not to mention later on when you get Fel-Armor, it applies to your Cannibalize as well, I tend to get 1200+ ticks off of my cannibalize nowadays.
WotF almost needs no introduction, considering immunity to Fear by itself has turned many tides in my battles, even saved my party's wipes on occasion. Top notch racial if they're ever was one.
Quote:
Blood Elf:
Main contributing racial(s): Mana Tap, Arcane Torrent.
Now Arcane Torrent aside, Mana Tap (the ability to gain a little bit of mana from your foe) is OK but nothing groundbreaking, especially considering how locks can regain mana on the fly from other sources.
Arcane Torrent itself can be devastating when facing casters, but with limited range (Hard to get close to casters when they dont want you close) and soso mana return almost makes it more trouble to use in a volatile situation in PvP.
You can also take into account what you want the character to look like, if you choose an orc, prepare for hilariously-proportioned shoulderpads (Although when the Dev's "Did us a favor" and shrunk them for a patch or two it was even worse looking) or undead and thier gear always physically looking like it should be in red status, I would suppose Belfs would be the Asthetic choice as well, but then again if you rolled horde, chances are you didnt roll for looks.
Sorry guys, its been a while since I posted, Ive been adjusting to pre-med.
G'luck.