Here and there, I would have liked it if the episodes were even a little more adult-oriented - the series pulled a few punches for the sake of a mixed audience. I don't mind the fact that nobody EVER dies in the series (the worst to happen so far has been permanent insanity) but they set up some heartwrenching plotlines, and then use simplified resolutions which take a lot of the sting out of them.
I've been getting ready to watch the other DC cartoon series of that era to follow it up - Justice League (and JLU), Batman Beyond (the reason I wanted to rewatch B:TAS was as a precursor to rewatching BB), and then Teen Titans, which keeps some of the same atmosphere, despite its zanyness and manga style. Given I'm planning to follow it through that far, it would probably be remiss of me not to watch 'The Zeta Project', which I only realised recently was a spin-off from Batman Beyond.
I'll probably watch them in that order - aside from Teen Titans, that keeps it fairly chronological by plot. It's possible that JL runs partially concurrently to B:TAS (why couldn't they have called it Batman Animated: The Series? I'd make fewer typos!) but I don't think it does. Teen Titans is divorced enough from the rest that I don't think it would be worth watching in order - and besides, it'd delay me watching BB!