Since downloading the bitcoin client could be a problematic with the increase in block size (even now it is troublesome), would there be an easy way to enable users to specify any bitcoin addresses, including within B&C?
This question might be answered by BCExchange in more detail, but according to the
BCE design (page 3) users should be able to deposit funds at multi signature deposit addresses without having to have an up-to-date Bitcoin blockchain.
The foreign blockchains (BTC, LTC, whatever) are handled by the reputed signers and they create individually per user account multi signature addresses if users want to deposit funds.
That's why using BCE will be as comfortable as using a central exchange but the funds are even safer at BCE, because they are protected by the reputed signers of BCE
That's why it can truly be called decentralized exchange!
Remember, there is no single point of failure; there's no website that can be DDOS'ed, no Mark Karpeles that can mess with your funds, no single point that can be attacked, no way to get your funds seized by a government going nuts about crypto.
It's only the users, multiple reputed signers and the blockchain