Which should we work on as the unsolved problems in cryptocurrency and why?
Scalability - Increasing transaction processing capacity and speeds on blockchains to support wider adoption. Issues around block size limits, transition to proof-of-stake, sharding, layer 2 solutions.
This is a major problem confronting Bitcoin and I think it has been used as excuse by many businesses to reject Bitcoin payment. It came second, after volatility, as the reason many people are reluctant to accept Bitcoin payment.
Privacyy - Enhancing privacy protections for users on public blockchains while meeting regulations. Zero knowledge proofs, confidential transactions, mixers/tumblers.
Mixers/tumblers are doing fine for this but I don't know how these and regulations can work together.
Security - Preventing hacking attacks, thefts, fraud on blockchains themselves as adoption increases. Consensus protocols, key management, formal verification.
The greatest security threat is found in CEX and there have been several post here discouraging people from saving their funds in CEX. Remember "Not your keys, not your coins".
Others - We welcome your own ideas/pain points
What I want to add is the need to create more awareness even from our immediate communities. The more people know about Bitcoin, the better for us.