When debugging multi-threaded application, I found
that showing threads and call stack in separate
windows makes debugging of such an application
difficult, if not impossible. The problem is that
you don't see 'movement' behind scene. It can
happen that you are stepping somewhere in the code
and you unexpectedly lose focus. 
And you don't see suspended threads at once. What
could happen? E.g, scheduler caused switching
context to another thread or another thread was
running and now is waiting somewhere in monitor
holding the same resource as your debugged thread
requires.
It would be nice to see all running threads in a
plain list (or tree with possibility to expand all
nodes by single click) with call stack under those
ones in waiting state. Something similar to this:

-session xy
  -thread a (running)
  -thread b (running)
  -thread c (waiting)
    -ClassLast.methodLast():143
    -ClassLast.methodPrevious(Param1Type):567
    -ClassPrevious.methodPrevious():23
  -thread d (running)
  -thread e (waiting)
    -....
  ...

etc.

In this way of showing threads and their stacks,
you can easily switch between threads and you can
easier and faster find the thread holding the
resource you require or see newly created and run
threads.