There shouldn't be anything else.
In prior times there used to be issues with Freesync and similar technologies, but these work with borderless (windowed) fullscreen now as well. On DX12 (so, on a modern operating system) borderless fullscreen should work / and normally does work just as well as exclusive fullscreen. If the game is not an old one that does something silly on task switching, there are no issues with multiple monitors either (some of the older games manage to lose the device and / or minimize).
Overall, the move to DX12 makes absolute sense even given that it does not bring immediate performance benefits. WoW isn't doing bad there. It is hard to expect performance benefits this fast, you have to get lucky to get them that easily. It will pay off in the long term multiple times over and a typical scenario will likely be them doing something cool and flashy and helpful and without compromising performance too much, and then a week later someone sitting in the corner of a dev room will remark "btw, would you imagine trying to pull this without DX12?" and everyone will go "yeah, we wouldn't even dare" and nod.