A Vanilla and TBC legacy server should be fairly non problematic to implement, providing people don't expect any modern improvements WoW has currently.
But from Wotlk on, that's when the problems arise. LFD is almost certainly much different than it was back in WotLK/Cata/Mop, Blizzard will have to find some way to implement the current version of LFD, or somehow implement LFD the way it was those expansions without fucking up something.
It gets even more complicated with the account bound systems and transmog. People will want those in their current state.
These are big reasons why Blizzard is not too sure about legacy servers. There's a lot of huge technical challenges and compromises that have to be solved first.