Only trial accounts have player interaction restrictions. - Edit 1
Before modification by TaskmasterJack at 19/05/2011 11:05:20 PM
Or can a vanilla WoW player run around with someone who has WotLK? Also, aren't certain races and classes expansion-specific?
You have to have Burning Crusade to be a Blood Elf or Dranei. I think you need WotLK and a currently 55+ character to make a Death Knight (though I'm not positive). And you need Cataclysm to play as Goblins or Worgen.
But you can still interact with those players who have the expansions when you have basic WoW. You'll just be stuck maxing at the old Level 60 level cap until you upgrade. And you won't be able to enter certain "expansion content" zones.