There are a lot of sites on the internet where people can post links to things or questions or even make statements. Generally people are allowed to vote in different ways to create a ranking of interest.
There are a number of alternate systems for identifying reader interest. Reddit and Digg (originally) allow both up and down votes. Hacker News only supports up votes. Slashdot has no voting at all and posts are chosen by moderation. Facebook has likes which are up votes but since the items liked are not really shown together it’s not really in the same category.
To me down votes are a terrible idea. They give users—particularly the malicious and the deliberately manipulative—way too much power to influence what posts rise to the top.
The ideal of such a site is that posts which are worthy of people’s attention should naturally rise higher than those of limited appeal, spam and propaganda. Unlike a political election, where you can cast only one vote for each office, a link site allows you to vote once for each entry but vote for many entries. Voting is continuous and the ranking of posts is constantly changing over time.
Sites that allow both kinds of voting have a very different dynamic than those with only up votes.
Each user in an up/down system can cast only single vote for each post they like, but there is no defined limit on how many down votes can be cast for other posts in relation to the number of up votes. For example you could up vote a post you like while simultaneously down voting 10 other nearby or related posts.
This two sided voting system allows for a lot of potential manipulation. For example a social media marketer could hire a number of people in different parts of the world, post something for a client, then direct the captive users to promote this post but down vote other unrelated new posts. This increases the momentum of the sponsored post. Of course most sites attempt to identify such manipulation but it is not so easy to distinguish well-designed votbots. It isn't easy to know how often this is done on link sites but based on my experiences in the App Store I am fairly sure it isn't rare at all.
Of course down voting can also be done for other reasons. People often down vote posts based on titles, subject matter they actively dislike like atheists or religious people attacking each other’s posts and politically related posts are always fair game from opponents. These types of attack votes can alter rankings without regard to the post content’s benefits or interest. Being able to down vote in bunches gives more power to the negative votes—you could simply down vote anything you dislike without ever even promoting anything.
I prefer the system at Hacker News, which only allows up votes. In this way you can only promote posts; while you might consider not voting at all equivalent to a down vote it has far less influence. Even if you want to use a votbot to push up your post since you can only vote one way, your influence on unrelated posts is muted. Other posts may have their attractors who can still influence the ascendancy. Naturally enough up votes on a single post in a short time may push it up faster but you have no guarantee it will be enough. If there were competing votbots their effectiveness may quite limited.
On the app stores (I am most familiar with Apple’s) deliberate manipulation of both ratings, downloads and reviews is fairly rampant. One company I won’t mention has the same source code and content behind several of its brands yet the ratings quite different especially on the original main brand which is always highly rated and has two kinds of reviews: 5 star “this app is awesome” and 1-2 star “this app sucks”. The brands not promoted by the app ranking companies have all lower ratings and more common reviews. I remember seeing one review that admitted they only made the 5 star review because they were paid for it (which is clearly against the rules for this sort of thing).
The rule is that rating systems that can be manipulated for profit or attack will be. When I worked for a travel company (now just a brand of someone else) we had a secondary app which was fairly low in its category and someone decided to spend $10,000 on such a ranking company to see what would happen. For 3 days this app shot up to the top of the category but I tracked all the users and virtually all were in third-world countries we didn’t do business in. Once the money ran out the app dropped back to its normal place. So I am sure these types of companies are happy to manipulate any system they can for money.
Having a system which is hard to create enough leverage in is likely to produce a more organic ranking of posts. It's not a guarantee but with less ability to push down the competition it makes this a less interesting target.
You could argue that allowing downvoting provides more freedom to the user but I don't think it overrides the “downsides” to it. Downvoting in comments makes more sense to me since you really want to eliminate trolls and other people who only want destroy the conversation. Here there is less need to avoid manipulation (though of course not completely) with downvoting—if you vote everyone down but your post there isn't much of a conversation!
A variation I haven't seen would be to require that you down vote each period of time only as many times as you up vote. I don’t know if this is better, but it would seem to balance out better. It might wind up being more like an up vote only system.
Elections are a vote up only system generally. For each office you get one vote. I wonder what it would be like to allow single up vote but also allow a single no vote for the entire set of candidates; if “no” wins you have to schedule a new election for that office with all new candidates. It might even get more people to vote. Would it produce better elected officials? No idea but at least you might see some new faces.
Voting and ranking systems on the internet are complex and not it’s not easy to identify manipulation. My take is that simpler and more difficult to manipulate systems are likely to produce a better result.
You can, of course, disagree and vote that way!