Compression, conditional GET
Both are important for lowering bandwidth use. If I had to rank them, I’d say conditional GET is more importantâ€”but I find that there are lots of sites, even some very popular sites, that still don’t support conditional GET.
It looks like most (if not all) of the popular weblog hosting systems support conditional GET, which is very cool. It appears to be mainly custom content management systems used by one or just a few sites that tend not to support it. You can almost count on lack of support if the RSS or Atom feed is generated live by a script instead of being served as a static file.
(Note that I intend no disrespect for people and organizations that develop their own systemsâ€”I myself use a custom system written in PHP.)