being a p-plater myself, when i say this, i am not biased.
I think power/weight ratio is def the way to go. there should be stages, starting out with a level one, and progressing until a 3. You can only progress a level if you have no major accidents, large fines etc.
Level 1: cars on the slow side to start with, drive that for 2 years.
Level 2: upgrading to mildly powered everyday cars, drive for 1 year.
Level 3: turbocharged, 8s+, rotaries
Now i realise everyone is going to say although someone will get a "Level one" car (eg. Corolla), they can still modify the ass off it so it goes quicker. but hey, really your average teenager isnt going to bother with that. Although this would be a very difficult concept to police, as they cant estimate the "level" a person can be at as they drive past. it is only when they check licenses when they pull the driver over that they can determine if the car falls into the catagory. but hey, there will never be a concept that will please everyone.