Users have to create, share, distribute, and manage partially signed transactions. This is not viable for almost all users. Only the most paranoid and patient of users. The infrastructure required to "get this right" is significant.
As a general policy I think there should be no situation where a user cannot sign and submit a valid transaction to the blockchain. Multisig is a generic tool employed by Bitcoin to solve any arbitrary voting policy on spending funds. Being generic means it is harder to use.
I think we can identify the 2 most common use cases and make it even better than Multisig.
These use cases are:
2) transaction confirmation via 2 factor authentication service
The 2 factor authentication service is much more complicated and will require full use of multi-sig along with a "bot" that will automatically transfer all incoming TITAN transfers to unique multi-sig accounts.
In the case of escrow:
1) Sender Key
2) Receiver Key
3) Escrow Agent Key
The escrow agent should only have the power to divide amount between the Sender or the Receiver and no one else.
The Sender only has the power to give the funds to the Receiver.
The Receiver only has the power to give the funds to the Sender.
In this case we can create a simple state transition on who controls the funds and any user can create/sign a transaction that will update that state transition:
1) Initiate transfer
2) Sender confirms transfer *or* Receiver returns transfer
3) *or* Escrow agent resolves dispute... and divides the funds between the parties.
I believe this simple escrow operation is much easier for users to interact with than multi-sig. It would involve selecting an escrow agent at time of transfer and then clicking a "release funds" when the goods arrive. On the receiver side they can click "return funds" at any time. In the event of a dispute they both have to contact the escrow agent and pay for dispute resolution.