Ideally X/Wayland/Mir work so far under the hood that you won't notice anything. That said there will be changes if you've been using things like screen captur

Ideally X/Wayland/Mir work so far under the hood that you won't notice anything. That said there will be changes if you've been using things like screen capture programs (x11vnc, ffmpeg to record your desktop, etc.). There's also the probability that when things first shift to a distro using Wayland/Mir for the first time, things WILL break, so unless you want to be a beta tester I'd avoid it for the first few months (or more) until things get more stable. After that period of introduction though, what you should be able to expect is better ram/cpu usage for the display server (wayland's architecture makes it much easier to offload everything to the GPU, X has some warts that makes that very hard). Based on what I've seen there should also be better support for tear-free video playback and eventually possibly even proper NVIDIA Optimus support (and AMD's version) which should help a lot of people with battery life.