One reason I like MN, whilst you can 'react' to a post it doesn't affect the flow of conversations or push some to the top of a page over others. It's still dependent upon people giving a post attention to get it seen, but every post on a thread at least has an equal chance of adding to a conversation naturally.
I have always read Reddit and enjoy subs about food/recipes, watercolour painting, history, books. I only joined and made a personal account 6 months ago and quickly noticed the karma system was berserk!
I don't follow any subs that argue or contain drama, but almost any polite contribution I make gets downvoted to hell 
.
One example ~ A poster asked for advice about where to locate good quality white bread, so I mentioned what I'd found whilst shopping, where you might find some nice bread, seeded or unseeded, blah blah, and how freezing was a great idea to save money, etc. I just offered a benign encouragement to enjoy/simplify shopping. I was downvoted with gusto, lol.
Similar on an architecture sub, I mentioned some features in a town where I grew up and how much the built environment had altered in 40 years ~ downvoted into oblivion.
It isn't just me, there are countless downvoted comments that are either friendly, useful or at least completely benign.
I think these systems actually discourage communication, whilst enabling those who 'game' them to rise to the top. Many people on reddit admit to curating their posts deliberately to avoid downvotes, or to using titles with key words that will ensure popularity. It might not seem like a big deal, but at least online, this is a pretty bizarre way to foster discussion and useful info, not to mention how it can isolate and cheapen broader discourse.
I mention reddit but this system exists throughout most social or communication media on the web now. I see less potential for valuable communication with this trend, and wonder who or what it benefits, because I strongly doubt that it benefits actual people. I guess whatever generates the most attention is king, but I still can't wrap my head around the likes/upvote model at all.