Of the top of my head:

1. Simple signup

2. Onboarding to keep you out of global (follow topics) > follow people based on topics selected.

3. Fast client that doesn’t glitch all the time. No extra long loading times.

4. Further discovery is easy - new topics, new people.

5. Functional search.

Maybe I’m missing something but if clients nail those 5 they’ll more likely retain their new users.