There was a question on the Wiki Page, which might be of interest to some here...
Hi Wayne Warthen,
HCT is like HC but it accepts TTL levels.
HC is slower than LS, but it takes much less power. It's output voltage swing is much wider, but without a lot of drive current. It's ok to drive some LS chips with HC output, but your "fan out" decreases a lot.
It's not ok to drive HC with LS output, because LS does not reach enough voltage for the logic 1. Here is the difference of HCT. HCT inputs will work with LS outputs. I am not sure about HCT output drive current... I am inclined to think it's as limited as the HC output.