Hi Sami!
1. Stream is a hosted service and does not support storing log data in MySQL. MySQL has major performance limitations when it comes to running complex queries on large data sets and is not well-suited for storing logs. Stream is similar to services like New Relic, but for WP Admin logs.
2. We use WP.com SSO (Single Sign-on) because it prevents us from having to store login details for any customer and customers don’t have to sign up for yet another account somewhere. The process is the same as services that use login with Facebook, Twitter or Google.
More info on that here: http://wptavern.com/interview-with-stream-project-lead-frankie-jarrett
