1. The feeling that "this matters". Usenet wasn't just one discussion forum among millions, it was The One. 2. The life of the mind - the all-text nature of the old Usenet was somehow incredibly conductive to treating people as pure intellects. "On Usenet, it doesn't matter if you're a dog." Somehow even forums like HN don't capture that feeling today.
Also, a decent newsreader is still ahead of all modern browser-based software by a mile when it comes to following many discussion threads. For me, nothing today compares with trn.
Yeah, I know Usenet is still out there, and I can even be found there sometimes, but it's not the same feeling anymore, not of intellectual wonder and horizons and of being a pioneer. Probably, that's hard to explain to the youngsters who weren't there - wow, the internet has produced its crop of old-timers.