Multi-file regular expression find & replace functionality, with nameable saveable expressions
That's the killer-app feature for me. I could actually say that I think it's BBEdit that gave me my first glimpse of the power of Perl.
I use BBEdit literally every day for both HTML and Perl. It's got a nice plug-in architecture for compiled things like BBTidy, HTML tidying specially compiled for BBEdit, http://www.denison.edu/websrv/tutorials/tools/bbTidy.html, but I do Perl tidying via it's unix script architecture, whereby you can just add PerlTidy to a certain folder and then access that script from a menu.
P.S. The science fiction author Cory Doctorow wrote on a blog the other day that he's written all his *novels* in BBEdit, which seems rather strange to me, but what the hell, it's another note in its favour.
