What do you do when you get Ninja-ed?

[LYQ]

choose a brickwall. break it. with your fist. tell the mofos who's the man.

[Viorus]

Let other people know who did it so they avoid grouping with this person in the future.

[Mauda]

If the people I'm in group with, wont kick the person, I'll probably leave. Also put the person on ignore, so to never group with him or her again. But define ninja'ing as a lot of gear aren't very specific - agility cloth, agility+int, etc etc. I'd totally forgive say a Mage for rolling on Spirit gear (I'm a Priest).

[Norjak]

But the basic question is... is there any consequence for loot-stealing?

On a single realm, the consequences can include developing a negative reputation, making it harder to find groups & guilds after a while. Some people have been known to go so far as to report the player to the GM of whatever guild they are in - assuming the person did something which affects the guild's reputation on the server.
But yeah, if you join a group you are essentially trusting the leader will do the right thing when it comes to loot. If you don't trust the leader, you are free to leave the group.

[Aunstic]

If you don't trust the leader, you are free to leave the group.

Or make your own group.

Sometimes you get perks with leaders though. I attempted one on my alt. Tell everyone in the group that if I can be the only one to need roll on the Scarlet tabard, I'd give everyone 5g to pass.

That's with setting /script SetLootThreshold(1);

[FrustratedMoth]

I haven't gotten ninjaed yet, fortunately, but it'll inevitably happen someday.
If the ninja needed something everyone else greeded/passed, needed something for his/her offspec, etc etc etc, it's not a ninja in my book.

If they need something (for money) another group member needs more (read: CAN USE) i.e. a warlock needing on a 2-handed hammer the paladin wanted... That's when I get salty. It's the same concept as stealing someone's new car so you can sell it.

If the ninja-er in question responds with mouthbreathing nonsense (anything including kappa, fgt, loser, scum kill urslef, lololololo, qq, xd, etc etc etc etc etc) will make me screenshot them and post it to the forum.