Just so I understand your terminology, by "user mode thread" do you mean that you will have threads at the Arc language level, and that they will be implemented by your runtime by having your C code itself switch between threads? (And thus the Arc language level threads will all run inside of one operating system thread?)
One thought that occurs to me is that the next thing that might be most useful is to have a way to easily see the performance of the garbage collector in a program you're running. Then when you notice "oh look, Arc is allocating a large number of short lived small objects" or whatever you can tune your garbage collector (or look for a gc algorithm) that works well with the pattern you're seeing.
