Most of the sites want a custom banner and are, in fact, quite disappointed in them being made shorter because it means less of the site's unique qualities are retained and it means the sites are less distinguishable from each other outside of the logo itself. Sites like AU don't have as much art as sites like Photography or Science Fiction & Fantasy, so it's a different argument. I'm not sure if it's possible to meet the needs of both groups of sites.

The header being sticky is already an option in your preferences, actually. :) You can make it unsticky whenever you like. askubuntu.com/users/preferences/449 :D