Hey xDan!
Before I start responding to your points individually, I just want to remind you that coinb.in is aimed at the "advanced" bitcoin user and offers the "tools" required for them to understand how things work. Also the code is also remarkably simple and easy to follow compared to say bitcoinjs.
I haven't used the service, so I don't know if such a transaction would go through... But the fact it is even implied that it might is a very very scary thing and will keep me from using what otherwise looks like an excellent piece of software. Default to 100% of the value going to the transaction fee (if I don't put anything in the "amount" box) is crazy and might one day lose somebody all their coins.
A sane option might be giving a large red warning if the tx fee is larger than say 0.1% of the total value. Or not allowing it at all if it's over $1 worth or something.
The transaction would indeed go through if you signed it. The question is, who am I to prevent someone from choosing their transaction fee, regardless of how high or low it might be. It would be an unfortunate mistake to accidentally set a large fee so I will take into consideration what you've said and perhaps throw up a confirmation notice if the fee is too large. But please keep in mind this is the first time someone has mentioned this and made such a big issue about it.
If a change address is required, the user should be asked for it also with big warnings that it should be a safe, permanent address, not in a virtual machine or livecd etc (where people have lost large amounts in the past). A conservative approach to handling change is to send it right bank to the original address. Not quite as private, but better than losing money. (and still needs warnings to the user that the address/private keys they are sending from must be kept!)
Yes a change address is required, again who am I to send automatically send it back to them. My philosophy is its your money, you can do with it as you like.
If you dont want to use the tools to manually build a transaction, and want the comfort of your funds being returned to you, try the wallet instead
https://coinb.in/#wallet you dont get same functionality as building the transaction yourself, and signing it etc, but if the transaction fee is your issue, you can be sure it will return it to your address. Check it out
Thanks again for the feedback and suggestions. Very great full!