Kristian Høgsberg ended up removing alt-tab support from Weston last week. The early alt-tab support inside the Weston compositor was removed because it's fairly unstable, caused lock-ups with full-screen and exposay modes, and causes some issues if used on the same modifier as mod-tab. Kristian disabled it a short time ahead of the Wayland 1.4 release with hopes of having a new alt-tab feature for the 1.15 release.
While it's too late to be fixed up for Weston 1.4, there's new alt-tab code that was published on Wednesday.
This second-shot at alt-tab support in Weston is improved with over 250 lines of new code to Weston's shell. There's still some other cosmetic improvements that Emilio Pozuelo Monfort is looking to make, but at least it shouldn't be too long without alt-tab support in Weston.
In terms of the new features to the upcoming release of Wayland/Weston 1.4, there's many changes.