If I don't log in I'll miss out on something...!
That there is an 'in-built' luck system and you can waste good rolls and then get low ones when stuff drops. Back in vanilla (and when I was about 14) it was to the extent that I believed an employee from Blizz was watching all of your raids and decided when it was someone's time to win a roll XD
...This system seems to be the reverse of how I believe it works though. Back when I was raiding, it was a running joke concerning my notorious sub 15 rolls on every item.
i'm superstitious of joining pugs where the leader (~900ilvl) is asking for "Curve" and 930ilvl+. I'm afraid i will catch the jeebies.
before the infinite augment rune, every time you use rune after every good pull, the pull with rune up will be horrible (I have this tested )
The try after your best one in a boss that you wiped a lot will be like no one ever played WoW.
Your worst skill wise friend will get all the rare mounts/items first, like a DPS trinket he doesnt even know how to use properly.
If you havent played for awhile magically your loot chance seems to be higher.
Obvious superstitions:
I will never get the loot I want on my main, but my alts will be showered with every conceivable RNG gift the moment I start trying to gear them.
As a sort of corollary to the above, I only get loot when I don't really want it or am not paying attention.
This one so much. The 'melee' hunter in vanilla because they forgot to bring arrows? The 'melee' mage? The ret pally (in vanilla)? The mana sponge who by now with the amount of healer mana he's soaked could be classified as a weapon of mass destruction ala the Mana Bomb? The ultra casual guildy who probably won't log in again for a couple months that you just brought to fill the numbers?
All of these people will loot their BiS items even more so if you have a good dedicated player who always shows up flasked\Runed\Potted on time and plays well who also needs the same item...
When I was younger I used to hope bad things wouldn't happen.
Now I just hope they're at least funny when they do.