4042 is one of the first Milestone 7 builds, and it's instantly obvious. Sporting a new boot screen (see 4051) and welcome/secure desktop screen (see below), as you boot it up you can tell something has changed. A new wallpaper greets you, and shortly thereafter what was a brand new theme at the time - Slate.
All the M7 code has been checked in, and there's a dramatic increase in stability and speed. The shell is completely upgraded, bringing a much cleaner UI and better access to new features.
The amount of experimentation with features is kept low, only the most important Sidebar tiles remain in this build, and the "3D View" option in Explorer seems to be missing, thus making the Panorama and Carousel view styles inaccessible.
A new feature rears its head, stashed away in the WinSxS folder, StickyNotes. Unlike in the WinHEC 2004 build (4074), Stickies work off the bat by dragging an empty text file onto the StickyNote icon.
To top it all off, Glass has made the crossing to M7 unharmed, and is a lot more usable than in 4040. I give you, the first shots of the M6 Aero Glass borders applied to the new Slate theme!