It's clear that you've done a great deal of work to tidy up the login and the underlying code, which is much appreciated.
But as someone interested in usability, I've spotted a couple of issues which you might like to review.
1) When the screen first opens, you see the logo, the clock, and the user's name (I set up login during install). But there is no affordance (visual clue) to suggest that you have to click on the user name to bring up the login form. It looks the same as the non-clickable clock and logo. It wasn't immediately clear to me what I had to do to log in. This is every user's first encounter with Mint, and you want to make it as painless as possible. Baffling them with the first screen they see is hardly the best introduction. Please consider using a clear convention to indicate to users how to open the login form. I hate to say it, but this is an area where Win 8 is better...
2) The error message for an incorrect login pops up on an entirely unrelated part of the screen. This isn't user-friendly, particularly for people with vision issues. Generally regarded as poor usabliity practice.
The new login is so much slicker than it was on 15, but these issues make it hard to use. If you test it blind with new users (do you do usability testing?) I'll bet you'll find this is confusing people. If it takes them more than 1-2 seconds to figure out how to log in, you'll know you've got it wrong.