I’ve been out of the loop since 2.8. What did I miss?

In no particular order, other notable things are:

  • old default theme was deprecated in favor of annual theme changes, you jumped over Twenty Ten theme and current one is Twenty Eleven

  • WordPress MU was merged into mainline WordPress and is now called multisite feature

  • serious improvements to complex querying by taxonomies and custom fields

  • server requirements had been bumped to newer versions of PHP and MySQL

There had also been a lot of work done on admin UI and such. Too much of smaller stuff to list, just go through version pages in Codex (click version numbers, changelog pages are useless).