Major new features compared to 4.3.15 are:
* that this is now an "official" version endorsed by GNU
* engine-defined options can be set from a GUI dialog (for UCI and WB engines!)
* (optional) engine logos are supported in WinBoard
* it is possible to analyze obeserved ICS games with an engine
* the WinBoard binary is distributed as an installer including all essential support programs (e.g. Polyglot) and many optional extras
Some Screenshots:

WinBoard using a marble board texture and /autoLogo=true

Dialog to set engine options, using WB protocol extensions and a Polyglot that supports them