Ask HN: If you can port libc, you can pretty much run on anything right?

I've been having a look around the world of bizarre operating systems over the last day or so - particularly L4 and it's variants. It strikes me that if you can port libc to one's environment of choice then you've pretty much got it nailed in terms of running ... well ... anything.

Is this really the case or is there rather more to it than that?