The fact that almost anybody could make a Window Manager on X lead to a tremendous amount of experimentation and variety. Almost all of them were half baked and faded to obscurity, but it was a lot of fun to try them out. Also, if your bar is "better than TWM" then it's a pretty easy target to hit. These days the level of effort to even get to the baseline is way higher, one dude in a weekend can't try out some crazy idea.

Bit of a mix, but less diverse depending on the platform. Apples Skeuomorphic Era, was likely the height of "anything goes" in the mainstream.