1. Bitcoin (base layer): Privacy is still an after thought in most bitcoin wallets. Making UTXO management possible and easier to understand is a big hurdle it seems.
  2. Lightning: Failing faster. Failed routes are still a problem from time to time and that friction is something that will turn off new users
  3. Nostr: Client performance. All the apps are slow and use a LOT of network bandwidth. A lot of room for performance improvements and believe me when the masses come they will complain about this.