(Full announcement post tomorrow, but here's the changelist)
STRUCTURES
Added new type of block for structures: Wedge
Removed right-angle restrictions to blocks/rotation
Improved tools to make structure creation easier/faster
Added painted block support to structures
Added support for custom structure palettes per island
Increased hitpoints for blocks so structures are a little harder to destroy

ISLANDS
Added support for custom meshes in Layout: Ground mode
Import/move/rotate/scale custom meshes
Added support for grass/rocks on custom meshes (all types except Upper)
Added palette editing for islands (affects structures)
Sped up camera in layout editor (hold CTRL to go faster)
Added ability to create Normal/Challenge/Expert objectives for a custom island
Added list buttons for selecting an island in Sandbox mode
Added ability to turn off water for islands (height ceiling still active, but can go down really far)
Added Editor region as an option for islands (mostly for the grid-matching dust effects)

STEAM WORKSHOP
Added uploading/downloading structures thru Steam Workshop
Added Steam Workshop structure support to Island editor
Added uploading/downloading islands thru Steam Workshop
Added auto-downloading of Workshop structures on Workshop islands
Added support for playing workshop islands from sandbox mode, play mode, and more

VEHICLES/GAMEPLAY
Added variable max strength for part types
Increased max strength of many part types (things that spin or extend)
Made power wheels go faster/apply more force at higher strength values (can make faster vehicles)
Added reverse-cycling for strength (Hold SHIFT and press S or the button)
Added fine-tuning for advanced mode sliders (Hold SHIFT)
Allow disabling of effects for mini-thruster
Fixed collision with bullets and triggers, and collision with attachers + triggers
Reduced atmosphere falloff to be weaker, made balloon have way more buoyancy underwater
Changed map grid/dashed line drawing to speed it up