Update: A few more UV improvements that were not mentioned in the original post:
Viewport highlighting for UV selections
@Charles, this is now in: selecting vertices/edges/faces in the UV Editor highlights the corresponding sub-objects in the model viewports. (RH-72383)
Selection to model
A new button in the UV Editor lets you select UV islands and push that selection to the corresponding sub-objects in the model space (SelectionFromUvIslands). (RH-91095)
MatchUvScale command
New command to match texture scale across multiple objects — pick a reference object (or type in a value) and apply it to others, or use the average scale over an object. Requested here. (RH-89878)
ARAP now supports edge constraints
The As-Rigid-As-Possible unwrap method didn’t originally handle edge straightening constraints — that’s fixed now. (ABF still doesn’t support them.) (RH-87600)