I think the main difference comes down to the sorting algorithms. In Lemmy we get the organic content sorting done by collective human appreciation or lack thereof of said content (↑, ↓). Generally better stuff rises to the top, and worse stuff sinks to the bottom. You can still see either if you like by changing the order. That coupled with sorting by community does a great job at sifting through the noise. In Mastodon you have hashtags that can serve as communities but there's no organic sorting within that. If you subscribe to #Linux, you'll get pretty much everything with #Linux, whether one or a thousand people found it valuable.