I want to bring forward a discussion on how to use ETH to create applications. How would for example a user login to a service? Is it like a special login for ETH apps? I found out that MetaMask provides a possible login api. I would love to have more web 3 content here. I might as well try to write something first.
My ideal solution is a PWA with metamask api connection via firebase. I wish firebase could provide api functions for ETH to make things easer.
Good luck on this! Web3 is interesting but suffers from some really difficult-to-use tools and frameworks 
For authentication, I would look into Web3.js.
Thank you!
How will it be with new users? How would they register? Do they have to register by them selfs to blockchain ETH and then login to my service?
Yes - they will use their wallets (for the blockchain the app is tailored for) to get themselves authenticated!
That’s not that user friendly for people who are new to having a wallet. My thought 
is that the user base is very low for any service.