It's interesting to see what is considered as hot. As already mentioned there's hot after set replies, which I guess means 'long lived' but not actually hot unless it's maybe magma stream hot (calling it 'lava' instead might be fun...). It's also relative to how well a community is popping off, brand new hot looks different from AJ hot which looks different to a board in its high of several new pages appearing per day. So it's interesting to see how it's implemented regarding that, if it can scale at all. But I tend to go with my own viewing of hot, which is a thread with relatively high numbers and keeps appearing on a new posts skim in a short period.
Rarely does an automatic flag do this justice and so the feature means little in terms of what is really popping off. The best metric would probably be replies in x period of time, the spikes caused when the thread goes beyond just people leaving their 2c and when two or more users actually find something to engage over. If the feature can capture that it's done its job. But if it's something like above after relatively low post caps without a time qualifier, then it's just amusing. I haven't seen this feature used a lot in the first place to be honest but it's never done much for me when I have. I'd welcome a nuanced example closer to what I describe.