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The "What's Hot" list is on the device. When you go to the Featured apps, it's a gray button at the top.

that's the global list, but the per category list that OP was talking about is on iTunes desktop only. So pretty much worthless.

Nope, press "Categories" at the bottom and select a category to see the "What's Hot" for said category.

Eh, I don't see this on my iPhone. Selecting a category (from the tab at the bottom) shows Top Paid, Top Free, and Release Date.

No "What's Hot".

I think the iPad and Mac App Store interfaces include it, but the iPhone doesn't.