One interesting fact about Spec M is that it isn't integral; i.e., the ring M has zero divisors.  This was proved by Poonen in 2002:

<A href = http://arxiv1.library.cornell.edu/abs/math/0204306v1>"The Grothendieck ring of varieties is not a domain"</a>

Re points of Spec M:  I suppose if you considered varieties over R instead of C, you would in addition have the map sending X to the Euler characteristic of X(R), though I've never seen this used.