"deep", to me, means "are lot of things in it, some of which are behind other things or interact in interesting ways that require discovery".

Shooting the centre ramp over and over in BDK to get Fox finished isn't deep, it's just tedious. BDK doesn't have lots of rules in it, all told. Is is well balanced? Mostly, so you see more of the rules than in lots of games where you do the same thing over and over.

I agree with the posters above that BDK's a fairly 'shallow' pin. A lot of the rule complexity of games like X-men and AC/DC is sufficiently hidden that people don't notice it, though...