Threading, mutexes, tokens...

This is a "I'd like to learn it, finally" post :)

So far, I've been coding userland applications (C, Java, scripting 
languages...), where I use mutexes to serialize access to variables, 
structures, etc. A (simple) example:

struct {
    mutex_t lock;
    int a;
    int b;
} my_struct;

void thread_proc() {
    while (something) {
       lock_mutex(data.lock);
       modify(&data.a);
       release_mutex(data.lock);
       do_stuff();
    }
}

void main() {
    thread_t t1 = make_thread(thread_proc); // create&start
    thread_t t2 = make_thread(thread_proc);

    sleep(1000); // or do something usefull
}

I see this is fairly different (trivially simple?) than situations 
reffered to in various posts explaining mutexes and tokens - 
specifically, I don't "see" the need for "With a hard mutex model you 
have to pass your mutex down into other subsystems that you call so they 
can release it before they potentially block, or you have to temporarily 
release your mutex yourself, or a myrid of other issues" (quote from 
Matt's recent interview). I'm not experienced enough, so could someone 
give me a pseudocode-example of this (prefferably in a real-world case?)?

How (and if?) would my example, and the complex one, be changed by using 
"tokens"? (prefferably, could someone show me using a similar 
pseudocode?) Do Dragonfly tokens have any effect on userspace applications?

Thanks!