The two major points from my perspective would be security (like restricting read-access to the clipboard and input devices) and multi-DPI (like multihead with a 1080p laptop and a 4K monitor - seems practically impossible to get right under X).

EDIT: But as always, if you're happy with what you've got, indeed why should you switch? I really don't get this idea of there being "the one-size-fits-all" and the urge to tell others what to do. A major point of Linux is your freedom to do things your way. OP is just telling others (who may have older now no longer relevant experiences) how much things have changed. No need to get defensive.