There will be a new rewards balance on each account, much like your normal balance and savings balance. So 3x "balances" for each liquid currency.
All rewards will be deposited into this new rewards balance, which will then have to be withdrawn in order to transfer.
The reason behind this change I've been told is for scalability, though I'm not 100% sure how changing which balance it goes into accomplishes that.
It's due to serialization. Presently, a consensus node that cares about "can user x spend y amount of STEEM" must process every single post and every single comment, in order, to know if that's possible or not.
Exchanges, for example, only care about your main balance for sending/receiving payments. They would not need to process each and every post and comment.
It's also a matter of parallelism; the balances can be calculated in parallel and only require synchronization when the rewards balance is transferred to your liquid balance, which should be about a 10x reduction in calculations for the liquid balance.
I would be interested to know if there is a good reason for implementing this because adding yet another wallet will only add complexity and confuse users even more, we should be aiming at reducing all these balances not increase them.
This is a required architectural change for the blockchain to support scaling in the future. If it doesn't happen now, it will need to happen someday for us to support mainstream user growth.
Too much interdependence between subsystems. The reward balance allows us to track account balances without needing to track content or votes.
What problem are you correcting?
It significantly reduces the amount of operations that must be performed to run the blockchain, which reduces the resource requirements that are necessary to run an instance of the blockchain. This is intended to help third-party developers who are interested in developing blockchain applications, as the barrier to entry (for having powerful hardware) is much lower.
It is a small inconvenience to the user, but it will be a big + for the network.
I'd like to hear more about the problem that is being solved.