I just think that Miller created his niche, which is pulp goodness. When he strays from that, he's also suprisingly good, like with Batman Year One.
But every genius or artist has their downfall of sorts. I think Miller, at this point in his career, is getting too comfortable with his own style. I've heard that 9/11 also changed the way he thinks about the world, and this sounds like a silly cop out, but that might have affected his writing too.
With me, Buck Rogers is PULP, but not in the Frank Miller way. Pulp in a Tarzan/John Carter of Mars style, matched with a leather football helmet and jetback. That's Buck Rogers right there; vintage style goodness. If Miller doesn't get it, then...he's not the right man for the job.