The engage needs to be straight - how many times do you see boring in (Ben Robbinson is the master).
Actually Robinson is one of the straightest looseheads going around. He's shorter than some of the 190cm tight head monsters used these days for lineout lifting, and he makes them pay for it by simply getting under them and driving them back over their second row. Which is not to say he doesn't drive in and up occasionally, but then I've propped against Tony Daly and that cunning old bastard did it to me when the ref wasn't looking too.
If you want to go look at some overrated scrummagers, check out Mako "Sidewinder" Vunipola and Tony "The Myth" Woodcock. Both must have gone to the Bill Young School of Perpendicular Propping.
As far as the rest of it goes, en_force_er has it right - trying to manage multiple blokes cheating their arses off in this sort of close contact is difficult. Especially if the ref has never packed a scrum in his life.
Assistants are useful to a degree - but they've got offside lines to watch as well. Look at the penalty Robinson conceded in Test #1. It was a clear case of Jones rolling his shoulder in and collapsing, and neither the ref (standing on that side) or the AR saw it, because they were watching different things.
I'd have to say that reffing a scrum is far more difficult than propping one.