When Reddit launched its "Front Page of the Internet" to the world more than a decade ago, its founders, Steve Huffman and Alexis Ohanian, knew they wanted to give users the power to rank and control what appeared on the site. But it took some time to arrive at the clean solution Reddit uses today: dual gray arrows appearing alongside every post title, one pointing up, one pointing down. These are the "upvote" and the "downvote," the user-driven, algorithm-enhanced engine behind Reddit.

Little-known fact: Before the upvote and downvote, there were stars. And, for a time, there were even BuzzFeed-like individual reactions to links posted on Reddit. The most notable was "WTF," which a user could click while reading a post. Doing so cued a prompt, asking the user why she felt this way--and nudging her to comment on the post. "Everybody hated it," Huffman told me in an earlier interview. That "WTF" system lasted not days--but rather mere hours, according to Huffman.

Over the early months of developing Reddit back in the summer of 2005, Huffman and Ohanian tested multiple options for user reactions. In the end, nothing more complicated than "Up" and "Down" stuck. As Ohanian explained onstage at the Web Summit in Lisbon Wednesday morning, "stars lasted about two days, because Steve pointed out that the difference between an article with three stars and one with five would be small. And I deliberately didn't want a thumbs-up [symbol], because it doesn't mean the same thing across all cultures."

If these minor design and UI tweaks seem trivial, consider the carefully plotted evolution of Facebook Likes and, now, emoji-represented reactions over the years. Consider, too, the weeks-long collective internet freak-out triggered almost exactly one year ago by Twitter's transition from a star to a heart to indicate a "favorite" tweet.

These sorts of user interactions with social media are seen as crucial for platforms, which constantly strive for more and more engagement. When asked whether Reddit would be tweaking its upvote formula, à la Twitter, in hopes of increasing engagement, Ohanian threw a little shade: "We are probably not going to be taking too many lessons from Twitter in terms of engagement."

Onstage at the Web Summit, Ohanian pointed out that in some of Reddit's communities, known as subreddits, moderators have chosen to eliminate downvotes altogether, in favor of an only-upvoting system.

Another San Francisco CEO in high-top sneakers, Product Hunt's co-founder and chief executive Ryan Hoover, joined Ohanian onstage. He explained that while much of the design of his site, which allows creators of companies or projects to post their idea and get responses from the site's readers, was inspired by Reddit, he chose the route of extreme simplicity when it came to user reactions to posts.

He found that "people would downvote any of their competitors" when given the option. "So we intentionally left it with just an upvote option," he said. Hoover noted that the upvote-only system infuses Product Hunt with a spirit of "positivity and empathy."

He also noted that, well, more complex systems for user reactions get unwieldy and turn off users quickly. So, for now, he's keeping the simplest version of the Reddit strategy. Only up. It doesn't get any cleaner than that.