I'm not sure if this is the right place for this, but this annoys me a bit every time I see it. 'Hotness' is just bad English, surely it should be 'heat'? I'm British, so perhaps you do things differently over the water.
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Hotness is a word in the English language, which is the noun representing how "hot" something is. It's very commonly used to refer to the instances of "hot" that are not related to temperature (which heat generally implies).
In our case, we're using "hot" as in "hot topic", a subject or topic that is on the eyes and minds of a large number of people. Something a lot of people are looking at. While its word origins probably come from the temperature (as have many of the forms of hot), it's still not quite as direct, so "hotness" comes out more direct than "heat" in the UI.