What do you feel is Bitcoin's missing link?
One thing that would be useful is a way people can redeem private keys to a destination address of their choosing.
It is a simple process.
One field is for the input of the private key (which can be scanned using a QR code).
A second field is for the Bitcoin address where the funds are to be sent (also could be a QR code scan). (or SMS or e-mail address and send the bitcoins that way, or through Coinapult's API maybe?).
So the web app (which works well from a mobile as well) gets these two pieces of information, applies a small handling fee, and broadcasts the spend transaction. (Use bitcoind v0.7 which lets you create and send a raw transaction)
Easy peazy. Access EasyWallet.org from a mobile to see a mobile-enabled web site that does 90% of the UI for this:
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http://www.EasyWallet.orgBut nobody has done this yet for redeeming private keys.
There are reasons people wouldn't want to use this, as that private key is now compromised (as you have a copy of it), so a person would never want to use this for an address that they might receive additional funds at in the future. But for something like a paper bitcoin from BitAddress.org, there is currently no easy way for someone to use that and redeem it and have the funds sent elsewhere.
You could even add features from there that make it even more useful. For instance, let the user enter how much they want to redeem, and then ask for an additional address for where to send the change. Or redeem multiple private keys at once, etc. There are all kinds of uses for a stite (and/or mobile app) that does this.