I’d argue that it’s not so much the free-wheeling discussion aspects, as the staggering variety of subreddits about any and all possible topics, sex and really weird stuff included. So while the SDMB seems to be more general knowledge based, with a sort of preference for politics, pop culture and the like, Reddit has high activity subreddits (and often more than one) for specialized topics- barbecuing, cocktails, beers, cosmetics, PC building, gaming, home improvement, and so on. Pretty much any niche interest you have, you can probably find it on there. Most major metro areas have their own subreddits where various local stuff is discussed, and when disasters or other big events happen, resources are shared/marshaled as best as they can.
I think the upvote/downvote is an interesting thing… not sure if it would work here with the current culture. And on Reddit, posts can often get lost in the static depending on how people are sorting the posts, etc… so it’s not as intimate feeling, and you don’t know if your post ever even got read, much less commented on.
If I had to describe it to really old farts, I’d call it the spiritual successor to Usenet newsgroups, just with a bigger platform and a better interface.
I do agree though, that there was a sort of… revamping of the moderation guidelines sometime in the past (probably about a decade and a half ago if memory serves) and there was a pretty hard crackdown on more vulgar thread topics like sex, excretions, etc… and IMO, that changed the tone of the SDMB for the worse; it went from fairly light hearted and fun, to something more serious and staid.
I mean, there were threads about stuff like “What was your worst sex injury” or “Tell me about how you pooped your pants” (I remember the threads, but not the actual titles). They were hugely popular and hysterically funny.