I think the aha moment (for users) is when they understand the scope of #nostr .. every client should somehow convey what is it all about !.. may be have links to multiple apps on the landing page when users get little comfy ..
As regards to cleint specific user retention .. I think every client has a core community.. #nostr shall be a collection of communities built around thousands of clients ..each surving a unique purpose but meshed across .. ultimately , every website would be a nostr client so that users could use keys and zaps across the web ..