One thing I didn't like when I tried Mastodon (and I'm talking about the web client implementation here, not the underlying protocol) was that the feed element was too thin and there was no way to increase its width without resorting to inspect element or grease-monkey scripts.
Mastodon thread: https://mastodon.social/@dsandoval/100137860645975194
Good part is that people actually replied to me, instead of the 0 interaction I would have gotten on twitter.