This is a fairly boring and standard stable release that's hopefully a fair improvement on top of Alpha 11 :-)
Mostly changes on the engine side are incremental, lots of UI improvements and general quality fixes. Lots of javadoc. An obsolete per-block liquid data type was removed, which might improve performance a bit.
Changes in module land remain tricky to summarize. The WeatherManager now has pretty particle-based effects for rain, snow, and hail! Nice to finally have that, although the current version struggles a bit performance-wise.
In part this is being done so we can do one more big round of impactful engine changes before the GSOC work period begins in 3-4 months, assuming we get in.