Great input skinbag.
To add to what you were saying...one other difference is in the manner in which you sit and "ride". The Harley-Davidson FatBoy is a cruiser. And as such you sit with your back straight up, like in a regular chair. With these "crotch-rockets" on the other hand, you sit leaning forward, way forward (with weight on your wrists). These crotch rockets also have a MUCH higher seat height giving the rider a higher ground clearance so your feet are not easily planted on the ground. It's this combination of (1)lighter weight (2)higher horsepower -- "Squirts" you forward (3)higher seat height and (4)seating position that will cause a rider to lose control easier than he/she would on a cruiser.
The throttle is MUCH more responsive on the sportsbike (you take off in a flick of your wrist) than on these cruisers. That's why for newbies, I'd say stick to something like the Suzuki SV650. There's plenty of power but it won't "jet" the rider off as a Gixxer or an R1 would.