One issue that I have
seen mentioned as an issue within the last week is still the problem of
running out of space when it still looks like there's space free.  I
didn't read the responses, so not sure of the resolution, but I remember
that being a "thing" with btrfs.  Is that still the case?  What are the
causes, and if so, how can we keep from getting a lot of the same
question on mailing lists/forums/etc.?

Josef gave a fairly detailed answer upthread:

In this reply, he does not specifically address the disk-full issue, but I seem to remember that it was resolved. I couldn't however find a reference---could someone authoritatively say something one way or another?