This’ll be expanded upon with some explanations.
Site license! – there are more users for this than you expect. (Note, I’m not ruling out open source here.)
Email from “Everybody” – no preregistration. Eg, HPC, bb-help.
Configuration and integration: extensible API, no proprietary little languages (where are my interface libraries to talk to MQ, etc, in the little language, if needed?)
Clever UI widgets.