humans: benefits 3% increased spirit = 3% more spell power from spirit. for pvp, the benefit by being able to have 2 dps/something trinkets instead of having to save 1 slot for escape trinket and a passive bonus to stealth detection
the reputation increase isnt as important as you think as it saves you 1-2 hours of rep grind per rep group and well, chances are you'll end up spending alot more than 1-2 hours killing/collecting the stuff for those groups anyways for 1 reason or another.. and once you're 999/1000 you might as well forget you even have the racial.
gnomes: 5% increased int - more mana, bit more crit and possibly more spell power depending on talent spec.. so all useful
escape artist - amazing escape spell. snaring and slowing effects are, imo, the biggest killer of warlocks along with stuns, so having a way to escape from them is incredibly worth while.
back when i made my lock.. pve and pvp ways, gnome was the way... now... sadly... human is probably the better choice for both.. BUT if you want the best for pve, you have to go horde and go orc
orc: 5% increased pet damage (very useful when as a non-pet spec, your pet will do roughly 15-20% of your damage.. as demo, anything up to 60% from what i remember.. (tis been awhile since i was demo)
blood fury - 2min CD racial trinket that gives you like.. 160-200 SP, so even more dps from that
hardness - 15% reduction to stun duration. well stuns are a ****** for warlocks, so having something to reduce them is always a bonus.