I really like all the new grammar stuff. Being able to write real parsers without having to actually write the parser is going to be great fun. I even heard that it would be possible to write a proper XML parser in pure Perl that would be reasonably quick and efficient, although I'm not sure what values of 'efficient' they were thinking about.
I think my favourite feature, though, is going to not be a language feature at all, but the profile of Perl itself. Perl6 will look a lot more modern a language to Perl5, which has a few crusty idiosyncrasies that we've learned to live with. I'm really hoping that Perl6 will bring new people to the language who may not have otherwise looked at it, and yet still be the Perl we all know and love.
The one feature I want to see in Perl6, which I'm not sure is there yet, would be a really easy way to distribute modules (possibly as Parrot bytecode?). CPAN is great, but I'm thinking more along the lines of what PAR is doing - being able to install cool modules (like Class::DBI ;) on your ISP's hosting environment by FTP, for example, would be amazing.