Imagine posting as a game scored in utility. Upvotes gain you utility; downvotes lose you it
That's exactly my problem with reddit-style voting in general. Human communication, even in an impoverished medium such as forum posting, is highly, highly complex and pluridimensional. Plus one and minus one don't even begin to cover it. Even when the purpose is a quick and informal moderation system. Good post on a wholly uninteresting topic? Good ideas once you get past the horrendous spelling? One-line answers? Interesting but highly uncertain info? Excessive posting volume? The complete lack of an answer where one would have been warranted? Strong (dis)approval looking just like mild (dis)approval? Sometimes it's difficult to vote.
Besides, the way it is set up, the system implicitly tells people that everyone's opinion is valid, and equally valid at that. Good for those who desire democracy in everything, but socially and psychologically not accurate. Some lurker's downvote can very well cancel out EY's upvote, for instance, and you'll never know. Maybe some sort of weighted karma system would work better, wherein votes would count more according to a combination of the voter's absolute karma and positive karma percentage.
To address your specific concerns about upvote-only systems, positive feedback expressed verbally may be boring to read and to write, hence reducing it to a number, but negative feedback expressed silently through downvotes leaves you wondering what the hell is wrong with your post and according to who. As long as people can still reply to each other, posters of cat pictures can still be disapproved of, even without downvotes. And perhaps the criticism may stick more if there are words to "haunt" you rather than an abstract minus one.
However, this one strongly depends on community norms. If the default is approval, then the upvote is the cheap signal and a downvote-only system can in fact work better. If the default is disapproval, then the downvote is a cheap signal. An upvote-only policy works best in a significantly more hostile environment.
Cross Posted at the EA Forum
At Event Horizon (a Rationalist/Effective Altruist house in Berkeley) my roommates yesterday were worried about Slate Star Codex. Their worries also apply to the Effective Altruism Forum, so I'll extend them.
The Problem:
Lesswrong was for many years the gravitational center for young rationalists worldwide, and it permits posting by new users, so good new ideas had a strong incentive to emerge.
With the rise of Slate Star Codex, the incentive for new users to post content on Lesswrong went down. Posting at Slate Star Codex is not open, so potentially great bloggers are not incentivized to come up with their ideas, but only to comment on the ones there.
The Effective Altruism forum doesn't have that particular problem. It is however more constrained in terms of what can be posted there. It is after all supposed to be about Effective Altruism.
We thus have three different strong attractors for the large community of people who enjoy reading blog posts online and are nearby in idea space.
Possible Solutions:
(EDIT: By possible solutions I merely mean to say "these are some bad solutions I came up with in 5 minutes, and the reason I'm posting them here is because if I post bad solutions, other people will be incentivized to post better solutions)
If Slate Star Codex became an open blog like Lesswrong, more people would consider transitioning from passive lurkers to actual posters.
If the Effective Altruism Forum got as many readers as Lesswrong, there could be two gravity centers at the same time.
If the moderation and self selection of Main was changed into something that attracts those who have been on LW for a long time, and discussion was changed to something like Newcomers discussion, LW could go back to being the main space, with a two tier system (maybe one modulated by karma as well).
The Past:
In the past there was Overcoming Bias, and Lesswrong in part became a stronger attractor because it was more open. Eventually lesswrongers migrated from Main to Discussion, and from there to Slate Star Codex, 80k blog, Effective Altruism forum, back to Overcoming Bias, and Wait But Why.
It is possible that Lesswrong had simply exerted it's capacity.
It is possible that a new higher tier league was needed to keep post quality high.
A Suggestion:
I suggest two things should be preserved:
Interesting content being created by those with more experience and knowledge who have interacted in this memespace for longer (part of why Slate Star Codex is powerful), and
The opportunity (and total absence of trivial inconveniences) for new people to try creating their own new posts.
If these two properties are kept, there is a lot of value to be gained by everyone.
The Status Quo:
I feel like we are living in a very suboptimal blogosphere. On LW, Discussion is more read than Main, which means what is being promoted to Main is not attractive to the people who are actually reading Lesswrong. The top tier quality for actually read posting is dominated by one individual (a great one, but still), disincentivizing high quality posts by other high quality people. The EA Forum has high quality posts that go unread because it isn't the center of attention.