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:

["The Grothendieck ring of varieties is not a domain"][1]

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. 

**Update**:  Oh, I've never seen this used because it's totally wrong.  For instance, A^0(R) and A^1(R) have Euler characteristic 1 but P^1(R) doesn't have Euler characteristic 2.  I think the mod-2 Euler characteristic would probably be OK here.

  [1]: http://arxiv1.library.cornell.edu/abs/math/0204306v1