Objects that shouldn't be in the established lore.
You wouldn't believe how many times my character on WoW had taken a quiet stroll down an avenue in Stormwind City and found that people are using items that are big no-goes. So let us sum up some of the things that we can imagine won't be in the game, but that people are almost moronic enough to roleplay into existence.
Cigarettes - Okay, first of all, look at this quote from the Imperial Library:
"The Wood Elves, of course, cannot smoke anything of a vegetable nature. Bone pipes are common, however, and are filled with caterpillars or tree grubs."
Skooma pipes & these kinds of pipes DO exist, and we even found tobacco leaves in Oblivion, but there's a VAST difference between the innovation of a pipe and a cigarette, just don't do it.
Coffee - Again, if we compare it to its real-life counterpart, coffee wasn't introduced and produced until the 15th century, and more than everything, it's a gimmick for people who roleplay characters that are tired and depressed. Go drink some mead to quell your sorrows!
Electricity - I don't even need to comment on this one. Only the Dwemer seem to have an understanding of thise (and even their exploits aren't too impressive when we look at them with modern eyes!), and there's no-one who has been able to duplicate any similar effect in lore. NO-ONE.
And that's just a few. Discuss, and post your own little things you believe shouldn't be in the game!