1) Every frame is perfect (Every frame is great). I.e. tear-free compositing in all cases.
2) Vast simplification of the whole system, including ability to run as a non-root user.
3) Potential to unify desktop and mobile graphics stacks (Android people as hell don't want to run X-server).
4) Fixing some old-standing X deficiencies.
Existing X applications won't be affected. X is going to be supported for the foreseeable future.