wait/arrive seems obvious to me. So I'll talk about that... As a synchronization primitive, its very useful for things like:

    parallel_memset(data, 0); // The 1024 threads go off and, in parallel + some efficient manner, initialize the datastructure
    barrier(); // wait for all 1024 threads to reach here before continuing.
    do_stuff(); // This probably relies on all default data being set before starting

Cluster arrive + Cluster wait allow for you to split the barrier into two units. Maybe the first half of a loop must be synchronized, but there's "spare extra work" you can do before calling wait(). This likely makes threading smoother.

    parallel_memset(data, 0); // The 1024 threads go off and, in parallel + some efficient manner, initialize the datastructure
    arrive();
    do_spare_work(); // some extra work before checking for a wait
    wait(); // We ran out of main work + spare work, so now we must fully stop and wait
    do_stuff(); // This probably relies on all default data being set before starting

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As far as the L2 cache thing specifically: I don't know.