New language

Runtime optional.
GC optional.
Static typing.
Binary compatibility with C.
Algebraic types.
Lambdas.
Traits.
Safe shared mutable state.

>>32
For one, the compactor is O(n·m), and it still doesn't solve the major collection scheduling problem. To get guaranteed better memory use, you'll still have collect more often than calling free/malloc.
Well, this discussion isn't about GC vs MMM anymore, but about naïve heap versus everything else; a GC can be (and often is, sadly) implemented on top of a naïve heap, and MMM can be made to use a more refined allocation strategy.  I guess my point really is that if you're going to use a more refined allocation strategy, you might as well slap a GC on top of it as well at almost no cost.