Twitter's 'quoted tweets' feature and how design affects behavior
Twitter's 'quote tweets' feature is back in the news in my circles because the Fediverse's Mastodon software famously deliberately doesn't have them. I find 'quote tweets' to be a fascinating example and case of how what looks like relatively neutral technical or design 'solutions' can drastically change the social side of a service. But to understand this, I need to cover the path that Twitter took to having quote tweets, because they didn't spring out of nowhere.
In the beginning, Twitter had no quoted tweets at all. However, people still wanted to do things like discuss things that other people had said or point their followers to something with additional commentary. So people did the obvious thing; they wrote a new tweet and linked to the original. Like this:
I find this argument for abandoning UTC leap seconds to be interesting but ultimately wrong-headed. http://twitter.com/auser/....
If you were sufficiently interested in whatever the tweet was talking about, you could follow the link and read it, but otherwise you were probably flying pretty blind. My vague memory is that people did this every so often but not very much.
Later, the web version of Twitter got a general link preview feature that I believe is (still) called 'cards'. If a tweet has a link, Twitter will put a little snippet, preview, or whatever of the link target below the tweet itself, so you can see something of where you'll be going before you click (and maybe you won't click at all, especially as Twitter will do things like play a Youtube video inline). Naturally, if your link was to a tweet, Twitter would basically inline the tweet in the card. This pretty much created the visual presentation of a quoted tweet even if you still created them by hand, and my memory is that this made them rather more popular since when you linked to a tweet this way, people could see what you were reacting to or commenting on right away, making it far less opaque and more interesting.
Then finally Twitter decided that enough people were quoting tweets this way that they would make it an actual feature, 'Quote Tweet'. Although the visual appearance of the result didn't change much (or maybe at all), the actual feature made it much easier to actually do, especially on smartphones (I'm not sure if it changed what notifications the original tweet author got, but it may have). Naturally this significantly increased use of the general feature and led to a situation where many people consider it to contribute to negative behavior (cf Mastodon's reasons for not having an equivalent). What had once been a relatively esoteric and little used thing suddenly became a common thing, even the source of memes (eg various 'quote tweet this with ...').
(I suspect that making it much easier to quote tweets on smartphones was a big part of increasing their usage, since my understanding is that a significant amount of Twitter usage is from smartphones.)
Each step in this evolution is reasonable and appealing to people using Twitter in isolation, and is probably not large in either technology or design (if you accept the general idea of cards). But the end result is a quite different social experience.
(I'm sure that this has happened in other systems. But Twitter's step by step evolution from extremely minimal beginnings makes it visible and fascinating this way, especially as the early people using it came up with many of the core ideas that were later implemented as features.)