DoomNineForum

While largely dedicated to DVD backup and other kinds of "ripping", the AviSynth Usage forum at Doom9 has a very high s/n ratio for technical people who dig desktop video. It's no coincidence many AviSynth developers hang out there (and the newer AviSynth Development forum); in fact, for any issues beyond structured bug reporting & feature requests, it's probably an even better place to interact with the development team than SourceForge.