Echoing many things that other users are saying already:

Signing up/choosing a home instance is confusing. I don't think it's very confusing conceptually, but it is confusing from a UX/UI perspective. Subscribing to outside communities was the toughest part, I had to find them through a different instance using a search engine, then manually paste the community-specific URL into my home instance search, wait several seconds, then click into the community home page and finally click "subscribe."

Not something a casual user is going to want or even figure out to do. I trust that many of these growing pains will be fixed in the coming weeks/months. I just hope that it's not all a flash in the pan and then fizzles out totally.

Once using it though, I like the general feel of it. Better themes and some cleaner UI choices and it will be really nice imo. People are friendly so far and that's worth a ton right there.