Features, screenshots, etc. can be found here as well.
I've used this client for 6 to 7 years now, and it's been in development the entire time (Which is a good thing, as it's far from being a dead project).
Originally the Perl scripting support attracted me, as well as multiple worlds within the same window. The built in spell checker is particularly useful.
I've also written my own 'colornames' plugin for it that automatically colors everyones names a random color to make it easier to distinguish names from the rest of the text.
MUSHclient ColorNames Plugin