The main change in J801 is to replace the J7 GTK IDE with one written in Qt, with better cross-platform support.
The J801 Qt IDE also includes a new version of wd, the J Window Driver. The syntax is similar though not identical to that of J6. It supports the main commands in J6, as well as new commands specific to Qt. See the wiki page Window Driver.
The J801 engine is the same as that in J701.
For users upgrading from J6, the J701 Features are also mostly relevant (with GTK replaced by Qt).