You are quite correct... it's possible to have no deposit, or 10X deposit for that matter - or once timelocks are implemented to have payments occur on a pre-defined schedule. So for example, if you are an unknown seller with an unknown reputation - it's quite possible that you would have to do a full double-deposit escrow contract to attract buyers given your lack of reputation... in time you wouldn't need that.
With smart contracts it should be possible to also have a 'normal' escrow account as well (provided said party was trusted by both individuals) with a multisig account defined. If either party fails to accept, but the pre-determined arbiter does accept both sides... then the contract is fulfilled and the deposits returned. As far as payment for services, etc. it would be easy to simply set up something where until cancelled by either party a specific amount is paid out per week... up to the maximum. If escrow is not involved and payment is one-way, then the remaining funds would be returned (at least as I understand the Halo whitepapers... I admit that I haven't actually attempted to code such a contract myself).
As with everything, it's just meaningless conjecture on all sides until we have the tools in our hands and can start trying out different options and providing feedback to David and the rest of the team as far as options/improvements that are necessary for our various prospective uses. It's quite easy to imagine that the purposes for which I am excited about using this marketplace are different from someone else's... it's also possible that once we've gotten more familiar with the interface and options - we'll have even more ideas for application which are currently unimaginable.
Definitely. Personally, I'm really looking forward to it.