I’ve written up a slightly more involved system for monster ‘specials’ that takes a page or two from Advanced (1st and 2nd). Basically I didn’t like treating all special abilities equally.
http://micahblackburn.wordpress.com/2012/08/01/house-rulesspecially-unequal/
I like your proposal and reasoning. It also bothered me (somewhat) that powers of different strengths have the same number of asterisks. One thing I do: i give one asterisk per two spell levels that a caster can use.
I think I mostly agree, although I would use HDx5 (min 20) for the damage cutoffs rather than 20 damage. A 10 HD creature doing 2d10+1 damage doesn’t have a special.
A ridiculous nitpick: A creatures that does 2d10 damage gets no ‘*’. 
Yeah meant that to be 20 or higher. I don’t think it should necessarily be based on the hd of the monster because a level 8 or 9 pc could still be wiped by 20+ hit. I’ll change that later.