The one issue that I think will catch people over and over again (and which I don't even know how to fix) is that if you find a profile or post on a mastodon other than your own you can't just follow or reply to it.
I know that there are browser extensions that make it work, but those are band-aids. You'd need an app that registered itself as the handler for every mastodon instance and opened all links in itself to handle it seamlessly for the average user.