I was positively surprised that Slang was actually quite usable. I had expected much worse.

I suspect the Common Lisp influence was beneficial.

The weirdest thing, language-wise, that I noticed was scoping. It's neither dynamic nor static scoping, but weird scoping. (But there are work-around to get something like static scoping.)

Outside of the language, the whole CVS-based version control and review process was weird. But understandable as a product of the late 1990s, when review-before-going-into-permanent-history must have been way ahead of its time.