Felix's mention of tables in my automatic margin request made me realize what another great feature would be. Having a grid or table of cells divided by lines is too tricky right now. What would be absolutely amazing is something that a lot of apps that deal with tables have:
At a container level, I want to be able to control the borders between its contents. This would eliminate negative padding hacks, as well as having to create "first" and "last" widget states where negative padding doesn't work.