In a recent Queue, Drindaar asked about expansion specific servers. Adam Holisky answered that Blizzard said they're never going to do it, but he thinks they'll have to eventually.
I think at some point Blizzard is going to have to do this, even if it means limited support and no GMing, etc... Even now, do you know how many people would come back to play on a BC-only server? An absolute fracken ton would.The next day, Matt Rossi said,
I don't think expansion locked servers would work or be a good idea at all...I'm more with Matt on this topic. I just don't think it would work. There are too many questions to be answered, and depending on Blizzard's choices, they may end up not pleasing the very people who want these kinds of servers.