Symphony.

Back in 1.7, Symphony auto-completed tag closures while editing xslt. Also, the tab key was captured which allowed for nice indentation. In 2.0, these behaviors seem to have disappeared.

Aside from here, I haven't found anything in the forums. Am I alone in missing these behaviors? Wrong in that they just need to be enabled?

You're not wrong - AFAIK they are not implemented in core. My guess is that it's planned as an extension. That way it could be used also for section fields maybe.

Er —I haven't even mastered writing events. That would be in line with the new model.

Yeah I'm sorry, it was pulled because WebKit changed its keyboard event model (for the better) sometime last year which unfortunately broke Symphony's auto-completion. I may be able to port the old version as an extension later this week if time permits, although I think a completely new implementation of XSLT auto-completion would be far more useful. (Maybe any jQuery experts out there want to give this a try?)