Plugins are where it gets weird. Technically there should not be proprietary plugins at all with the GPL, but it might depend on the interface or manner of linking (eg, would a HTTP-based "plugin" API be considered a derivative work?). For some cases, licensing the original software under LGPL may be sufficient to support proprietary plugins.
Ultimately it's a judgement call by the original author and what use-cases or freedoms he/she wishes to support. Hopefully they've thought that far ahead, though. It's not an easy question for the majority of the population that doesn't quite lean as far as RMS does.
