Having programmed both Hot AC and the regular kind, I'd say the biggest difference is in rotations. Hot AC's have hotter rotations, playing more current and recurrents in faster rotations, leaving less room for gold. The second difference is the playing of songs that are younger in appeal, generally bing daincier, rockier or otherwise less acceptable by the 45+ part of the audience.
The result is that Hot AC is more 25-44 and regular AC is a 35-54 format, and that is the distinction that most matters.
You can make the same argument about Active Rock and Alternative a decade ago. Sure, the audiences were slightly different, Active skewed a little older and played more gold, Alt played songs going up the chart, active didn't add them until they were going down the chart, etc. The reality was that they eventually wound up morphing into the same format. They played the same top 10 songs most of the time, and even spin counts became comparable.
The same thing is happening with Hot AC and AC. AC is becoming more contemporary and upbeat, and CHR is appealing to more of AC and Hot AC's traditional demos. Right now they are still three distinct formats, but I can see a day where Hot AC and AC are essentially the same format. AC's biggest calling card was that it was gold heavy and downtempo. That's really not the case anymore at a lot of AC stations these days.