Mint asks for your API key, which Coinbase implements rather poorly. Your API key will grant any application FULL ACCESS to your Coinbase account, so if Mint gets compromised, the attacker can send your Bitcoin to another account. And Bitcoin transactions are irreversible.
Yes, you do give Mint your bank account credentials, but if someone steals your username/password and use it to send a transaction, you can get it reversed.
Mint can easily fix this by using Coinbase's OAuth API instead of using an API Key, which allows for selective permissions (read-only) to be set.
I think if you are using an exchange your coins are not really in a known address. The exchange probably stores your coins, along with others, in many of its wallets.
However I do see how the ledger comes into play for your personal coin addresses. That is actually a powerful concept and pretty cool.
From a Mint perspective adding meta-data for coin transactions would be a good start to help categorize coin spends/receives. Another approach would be well known receive address sets. For example 'Grocery Store X' would have one (or multiple) well known 'receive addresses' that Mint could categorize appropriately.
At Coinbase, this is not true. You own your addresses. There is no "Hot Wallet". This may not be true of coins you just bought, that have just been added to your balance, but it's certainly true of coins you received as payment (to an address that you knew / gave to a customer / used in an API payment button on a website).
What Mint can't do without access to the API is know all of your owned addresses and aggregate their balances into one. Now as for the rest of your ideas, I'm not from California, but I honestly don't see any vendors signing up to use Bitcoins. Everywhere I go will take the Paypal card, but nowhere is offering to take bitcoin via NFC or QR code. Maybe "if you build it they will come."
I would like to have a way to export a global public key from my private key, such that customers could generate their own receiving addresses (for my wallet) without any sort of access to my private key, or third party. I have no idea if this is even a thing, but I also can't say for sure that it's not possible. If it were possible, I think it would also make it possible to pin the exact balance of a wallet, so I suspect it's not possible/an undesirable idea.
The coins in your address should be "mostly" visible to Mint - your address is still a public address even if it's hostd at Coinbase. However, that said, any internal Coinbase-to-Coinbase transfers don't hit the public blockchain (for a number of reasons) so if you have a lot of those then your balance would be incorrect. Any transfers from outside of Coinbase to your address or from your address to outside of Coinbase would be visible on the public blockchain.
Mint asks for your API key, which Coinbase implements rather poorly. Your API key will grant any application FULL ACCESS to your Coinbase account, so if Mint gets compromised, the attacker can send your Bitcoin to another account. And Bitcoin transactions are irreversible.
Yes, you do give Mint your bank account credentials, but if someone steals your username/password and use it to send a transaction, you can get it reversed.
Mint can easily fix this by using Coinbase's OAuth API instead of using an API Key, which allows for selective permissions (read-only) to be set.