The web interface wasn't good (worse than Twitter if you can believe it), the native client I tried wasn't good either, but the worst problem is that following accounts of a different instance was a major pain. In retrospect, I made the mistake of joining the smaller mastodon.online rather than mastodon.social, which they claimed wouldn't make a difference, but it did.
I couldn't just follow someone on another instance from their profile page, because their profile page is on a different server from mine. It wanted me to copy a URL, paste it into a search field, find the account, and then follow, which is ridiculous and tiresome.
The onboarding for new users on Mastodon seems terrible in every way.
(I also couldn't disable "boosts" in my timeline, which is something I could do on Twitter.)
I wanted to like Mastodon! But I just didn't, and I didn't feel like enough other people were going to like it either to make it worthwhile to use.