The mastodon social network (I'm https://appdot.net/@jgordon) lets me follow people at any Mastodon community (instance). Mastodon is person-centric. Reddit, by contrast, lets me follow activity on predefined topics.
I'd like Mastodon to have better topic support; I'd like to be able to follow both people AND topics.
In theory Mastodon has support for topics through hash tags. In practice, particularly if you are on a smaller Mastodon instance, the tags are not very useful. They only "know" about posts that have been pulled into a user's home instance, most often because someone on the instance follows the post author.
I'd like to see "topic tags" that were predetermined and worked across the Mastodon part of the Fediverse. I imagine a registry of topic tags that's updated by an instance daily based on instance posts using the topic tag. There are likely better models for how to do this.