If the product depends on a persistent relationship (ie- a subscription to WoW or even in the case of something like steam- You dont pay anything to continue using it, but the service depends on your continuing relationship) Then I think a EULA is binding- You are actually licensing something- The fact that the service is free isnt a issue.
However, the practice of licensing what should actually be sold will end. If you go to best buy and buy a copy of Tetris, for all intensive purposes that is a sale. There isnt any online environment thats needed. There isnt a subscription to it. Its just a purchase.
Anyways, to answer your question. I think the defining method of what will make something "eula capable" will be if it requires a continuing relationship. In the case of software, it might be simpler to say "Does this software require the internet to work" - If the answer is no, then its most definitely a sale and not a license.