That said, I never liked web forums in general - I prefer mailing lists (and the D-lang forum, as far as I can figure out, support usenet access (is a web usenet front-end) - so it should be possible use a native client rather than a web gateway -- which might be better.
But as the web has shown again and again, colourful backgrounds, smilies and avatars wrapped up in sloppy html and css wins every time - multimedia wins every time. Eg: slack.
I think it should be perfectly viable, and desirable, to meet both: an open (preferably federated/self-hostable) (set of) protocol(s) - a web client - and native clients. For me, the fact that I don't really thing gmail is any good, is kind of the only nail the "web app" coffin needs -- if Google can't make something half-way ok to work with in terms of UX -- for something that's arguably simple -- what hope does anyone else have?
As for your real question: I think wikipedia is a great example of both "it can be simple" and "complex provides value". Reading wikipedia works fine from lynx or w3m - but editing, viewing revisions, etc - benefits a lot from a richer "web app" client. (I would still prefer to edit in vim, use "real" version control and push changes - and I'm not sure what options are there for wikipedia, the project, or mediawiki, the software, to do that via apis etc -- but I'm sure it'd be possible to hook something up if it's not already there - I haven't really looked).