For a user: In Wayland programs are supposed to draw their own title bar. Java aplications and old applications must use a backwards compatibility layer that can cause flicker and bad font rendering. The terminology is different (compositor = window manager). Some niche new programs may only run on Wayland. Wayland hasn’t been adopted by BSD (AFAIK).
For a programners: Wayland has more modern, tidy code, but not all toolkits support it natively and few are easy. If you code exclusively for Wayland, a lot of users won’t use your program at the moment.