The frequency (training each muscle group three times a week) is the principle that makes HST so much better than most other programs.
Arthur Jones and Elington Darden in the 80's have recommended training the full body three times a week as well.
What makes HST significantly better than HIT, is progressive loading.
Training with maximum load all the time, will eventually lead to plateaue.
I find the most important factor (other than training frequently enough) to be eating.
(I agree with Totentanz on this one.)
I have to eat like a horse to make progress.
I believe training is secondary to eating (as long as you train reasonably):
Train great, eat so so = little or no muscle mass gain.
Train so so, eat great = fairly nice gains.
I actually find it really hard to eat all the calories I need.
It's really time consuming!
This might even lead me to spend more time cutting in the next few months, because I don't have enough time to eat...
Anyway, if I decide to continue bulking, I will consume twice as many calories from shakes as I used to,
(This means 1500-2000 calories from shakes...)
so as to make cooking and eating less time consuming!
(sorry for getting off topic, I just couldn't help myself after not having the time to reply for some time now.
by the way, my son is two months old today...
)