X11 is still server-first and needs workarounds to run locally (like startx, sx), while Wayland can just be run. Unlike X, it isolates every processes access to other windows, but with slow adoption of protocols for things like screen-sharing, video conferences, accessibility tools. The tooling is not yet there imo.
That's the main difference nowadays. Some people have issues with tearing or wrong-monitor with either of them.
