>> 1) No counterparty risk. Everyone can get hacked, but coinbase being hacked shouldn't every, under any circumstances result in me loosing any bitcoin they hold on my behalf.
> This point may prove to be very sticky. It might be solvable with smart contracts...
Shouldn't be that hard, even using just the features available in Bitcoin today. You'd need a 2-of-2 multisignature wallet for each account holder, with one key held by Coinbase and the other by the user. When the user wants to spend some coins, he or she signs a transaction which Coinbase prepares and counter-signs; neither the user nor Coinbase—or an attacker with access to Coinbase's keys—can transfer the funds unilaterally.
Shouldn't be that hard, even using just the features available in Bitcoin today. You'd need a 2-of-2 multisignature wallet for each account holder, with one key held by Coinbase and the other by the user. When the user wants to spend some coins, he or she signs a transaction which Coinbase prepares and counter-signs; neither the user nor Coinbase—or an attacker with access to Coinbase's keys—can transfer the funds unilaterally.
The only thing missing is a convenient UI.