Well, there's a few things - no long, enduring threads that aren't stickied, no image embeds, a general focus on short-form posts - but the most significant is probably downvotes. Why engage with someone when you can just click a little arrow next to their post?
Reddit stymies discussion by design, since controversial points of view - for a given subreddit - are pretty much guaranteed to be downvoted into oblivion rather than discussed and considered. If your opinion doesn't line-up with the majority, well, there's no point even trying to express it on most subreddits--even if you go to great pains to sound reasonable and open-to-discussion, people'll still just click, click, click away on that downvote arrow.
It's not something that happens to me often, but that doesn't make it any less crippling for discussion; I just deliberately avoid saying anything that's gonna get obliterated by downvotes. It's probably the biggest reason why I don't engage with Reddit nearly as much as I do other discussion forums. And that's speaking as someone who, generally speaking, tries to stick to the non-controversial discussions - cabin decor, for example - anyway! Well, unless something really annoys me.