Presumably the fact that you're tied into using Apple's window manager. But that's really loose and I don't totally understand why it merited being mentioned either, haha.

OSX fundamentally doesn't support robust tiling window managers. they exist only as a pale shadow of the real thing.

Presumably it's a sort of sideways/backhanded almost-compliment to OS X: it works well enough comparatively that its plusses overwhelm its negatives for the commenter. But, they still wish it supported more flexibility; and the fact it doesn't (and yet its plusses nevertheless still compare favorably enough to its minuses that the commenter continues to use it) leads to a sort of feeling of dissatisfaction and resentment.