Only to encourage the implementation of a STW(Stop The World)-free GC e.g. like brooks pointers:
I agree that a program can generally be made to work seamlessly through tweaks. Guillaume for instance has shown wonderful ability to tweak a program to not display any STW:s during an animation for instance.
You know this already of course though I just wanted to put it in words, while the argument that "STW:s can generally be tweaked away" is correct, then I suggest an STW-free GC gives more generality as a program will be STW-free without configuration or tweaks. Arbitrary application logics will be STW-free, whereas today only some are.
Utility is primarily UI:s and there especially animations, but also network interaction.
I'm fairly positive that recent computers with their bigger, faster caches and faster RAM and more CPU cores, should be able to run an STW-free GC with a small-to-unnoticeable footprint measured in wallclock time.