After Drupalcon, I made a To-Do list with some things we should definitely check out, try out or learn about.
Buy the new Pro Drupal Development book (second edition for Drupal 6), which is probably the best book available for any Drupal developer.
Check out the Firebug plug-in YSlow on our websites to see if we can optimize things more. A lot more front-end optimization tips from this presentation. (pdf or video)
Try Apache solr search on a website which could use it (lots of content). Inspired by this presentation. (pdf or video)
Keep a good eye on Acquia and their commercially supported Drupal. Watch their presentation here.
Check out admin-theme project Rootcandy and potentially useful admin-theme modules like vertical tabs. Download the pdf of this presentation.
Check out panels. The module keeps popping up everywhere but we still didn't use it anywhere. Getting the feeling I'm missing something...
Use the devel module more (effectively). Great tool for more complex projects. Provides an easy way to clear your cache tables for instance.
Keep on eye on the drupal.org redesign to see if we can help in any way. There is a twitter redesign group by the way...
Think more about usability in Drupal...