Not saying this is likely, but: what if this is an older less-featured prototype, or a design/features direction they're not going, or the low-end model of a batch of several new models? Leaking this could create complacency in competitors, or even effectively lower expectations among fanboys (making the ultimate release more surprising). Compare: Apple's misdirection on iPad pricing which helped make actual prices seem lower.

But what ace could they possibly have left up their sleeve?

OLED screen? Their competitors are already there and userland's reaction has been decidedly mixed.

I honestly can't think of anything else. Pony?

Larger screen/touch-area. More networks. PixelQi screen.

New form factors -- keyboard, flip/clamshell.

Surprising internal specs -- storage, speed.

Though I think it unlikely, an announcement of Verizon support would definitely do it for the US audience, good call.

More screen changes seem unlikely though [1]. The density/resolution is already a huge, attention-grabbing update. Apple tends to parcel those updates out. E.g. they didn't jump straight into HD video capture. They released a slightly-better camera first.

New form factors strike me as about as likely as a convertible tablet MacBook or an attached-keyboard iPad update.

The internal specs... Apple doesn't talk about those much. Surely it'll be even-faster with more flash. But beyond confirmation of an Apple-designed SoC [2], they wouldn't even mention those specs. Let alone hype them to the press.

[1] Does PixelQi even have samples at the pixel-density the update is boasting? Let alone at volume? I honestly don't know.

[2] Something that would have been spoiled by handing the hardware to Gizmodo btw