DASH build some amazing tools to improve fungibility, but I wouldn't say it's in the protocol...
I suppose that depends what you define as the 'scope' of the protocol. The masternode network are simply peers of the regular network which operate in a an alternate logic mode. Thats why they're often referred to as a second 'tier', or 'tier 2'. Their function is 'protocol level' enough to be subject to the condition of mining majority for successful adoption which I think is the significant criteria in that respect.
As far as I know, it's impossible to have a cryptocoin which doesn't have public balances for addresses
Doesn't Cryptonote do that ? In Cryptonote the balances are not public.
Yup, I think they go only by transaction number? Or am I remembering incorrectly? It's a super confusing Blockchain explorer. I decided above all else, that cryptonote will likely never work because the amount of information each transaction has to be run through is going to become such a large database, it will slow down so badly that it will become a slug. I can't see how they can possibly trim any of it. They say they can, but I have other issues I can't figure out, and I've tried. Still, no gui, a bloated block chain and no real developers working on it makes me think that if it's ever workable, it'll be 20 years in the future.
Being about 3/4 the age of Dash, almost 5 times the size blockchain, and far far fewer people using it (meaning not nearly as many transactions going through as Dash), their blockchain issues are massive.