I think it would be useful to have a unified homepage that displays a tailed set of questions (based on the user logged in) from across the range of stack exchange sites.
Something like a cross between the existing StackExchange homepage that lists top questions from all sites, and an individual site homepage that allows you to pick to sort questions by active/hot/unanswered/newest/votes/mytags etc.
Edit: As suggested by Lord.Quackstar an RSS feed of the unified personal question list would be good too.
Background: My usage of SO goes like this. When I have 2 minutes free I hit the unanswered page sorted by newest and check the top 5-10 questions. If one looks interesting I'll take a look. Since stack exchange sites have started to become active I now have to check SO, SU, Meta, Photography and Ubuntu, and sometimes PhotoMeta and UbuntuMeta.
What I would like is to go to one page and for it to show the newest unanswered questions from the sites I am active on. Possibly including optional filtering by tags (but for established sites like SO, I would want tag filtering, but for new sites like Photography I still want to see all questions as I haven't built up a good tag list yet as it's still stabilising).