600's aren't all like that. Yea the ZX6 and CBR600RR and god.. that new R6... etc. are all not exactly beginner bikes... but if riden correctly they're all safer than any harley, just because they're more manuverable, have better brakes, lighter, etc. and... occationaly better acceleration can save you.
But, as far as beginners there are plenty of 600's that could work pretty well, a lot better than a harley... like a SV650, a Seca 600, a FZR 600, etc. They won't have the crazy power bands the new line of hot 600's have. I'm sure there are a lot more 600's out there than could make good beginner bikes as well.
I'm not saying a harley isn't easyer to ride, I'm saying it's not as safe, there is a difference.
light weight in my opinion should be the number one requirement of a beginners motorcycle because it influences so much. With a lower weight it can stop quicker, turn quicker, and will be easyer to handle in general. Yes it will also accelerate faster, but it still won't go any faster.