looking at Reddit, HN, Lemmy, and lotide, it seems like there's a definite link aggregatorness that these sites/networks share that other sites/networks don't have. each post is structured in the same way: title, link (to an external website, to an uploaded piece of media, to itself), optional text. the feed is a simple list of posts, with no content displayed besides the title, poster, community it was posted to, and maybe a small preview. people can influence the order in which these posts appear through up and down votes. each community is semi-independently run and focused on a specific topic. comments are invariably displayed as a tree, and are subject to the same vote system as the posts
the content doesn't necessarily have to be links to external sites, but the interface is optimized for those and uploaded media and plain text posts are treated the same as external links
now compare this to Calckey, Akkoma, Mastodon et al. the interface is built around text posts, displaying them in their entirety. even if the post is only a link, or has media attached, it is treated the same as a text post ux-wise. no structure is imposed on the posts, so people can just submit them into the aether, rather than picking a community to post to first. posts are displayed in reverse chronological order, and there is no mechanism to influence what order the posts appear to others