Luckily, Stephen Greenblatt is a pretty jovial guy who prefers to collaborate rather than get himself embroiled in too many messy situations. Sure, New Historicism has its critics (like the very vocal Camille Paglia), but it's nothing a little steadfast "I'm not listening" can't remedy.
Nope: the greatest source of contention probably comes from good old New Criticism. In many ways, New Crit is the whole reason New Historicism got started in the first place. That's not to say the Greenblatt and company wouldn't have created a New Historicism without it, but it's always nice to have something to react against if you're a literary critic.
What the New Historicists were reacting against was the idea that the text stands alone, in isolated from the reader, from its historical context, and from all other works that came before it.
According to New Criticism, all a text needs is a reader, and it will be pretty much the same ideal text each time it's read. But the New Historicists ask: who is that reader? Are all readers the same? Where do they come from? What do they know? New Historicists insist that people are different and see different things when looking at the same text; it's their fundamental difference from the New Critics.
Looks like these two camps will never agree.