Why don't I get any CSS output?
In my case, I was getting this in the apache error log:

.../sites/default/files/.htaccess: Option MultiViews not allowed here


Checking .htaccess, I found:

# Turn off all options we don't need.
Options -Indexes -ExecCGI -Includes -MultiViews


Even though it's turning MultiViews off, it's still not allowed, and removing it fixed the CSS.
Enabling sites to set it int he global apache config would also resolve it.