OpenGL backported from 530 (Literally backported, nothing special)
New textures and materials system which uses raw PNGs instead of producing the texture in runtime.
Js5 master table has been fixed instead of being hard-coded into the client.
Uses native libraries from 742 for all OS, so HD compatability won't be an issue.
These are the ones on top of my head, there is probably plenty more.
There is a small memory Release in some tiles from a certain angle, they are coming from the animated models not being de-allocated immediately but instead it waits for the GC to call finialize().
There is alpha issue with trees and bushes, which causes edges to seem not seamless and have a gap in-between.
HD water have a small bug related to matrices, it is using the wrong matrix for some of the transformations, HD water is disabled by default.
There are still tiny connectivity issues that can be easily fixed.
I have not touched the server much in this source, I was mainly a client developer with weird requests to do.
Use at your OWN risk
This client is by no means finished or ready to host, if you don't know how to fix these bugs, then just don't bother using if you care about perfection.
The client is currently unstable and has few memory releases.