Yes, that would be quite practical for small communities. It may come with some limitations, e.g. is it possible to style/brand different categories individually? That is quite important psychologically and is probably not too difficult technically (it is standard reddit functionality).
Thanks for the wordpress discourse plugin pointer. Was not aware of it so now need to evaluate what sort of possibilities it opens up more generally 
The discovery of different communities would remain unresolved though. I think one of reddit’s key attractions is that it combines the forum / specialized domain focus with the ability to quickly discover and “join” a flexible collection of such forums (so each user has a portfolio of forums they participate). This creates a customized feed.