If you just say that's an exception though, there are ALOT of exceptions, to the point of it being a rule in itself. Just pointing out that there are a bunch of skills that were between 1 and 20 points, as well as a few at 1 point, so saying you had to have skills at 20 points isn't really correct. Without actually going and checking, I'm willing to bet most classes had at least 1 skill they didn't max(not 1 point wonders) when min/maxing their char. There are TONS of different builds that can clear hell mode with 8 characters. Most of the time you just needed the right items to augment the skills you wanted to use (Auradin, Charged Boltress, etc).
Yes you would never put less then 20 points in frozen orb if you were an orb sorc any many other skills, but there is a lot of skills that break the rules. Maybe you want some utility, so you put a few points in frost nova. Putting 20points into something may stop you from getting some other ability. At the end of the day I expect it to basically be the same as d3 as once you pick one ability it will narrow your choice of other abilities, 2 will narrow it more etc. How does it actually make much of a difference which system you use? D3 Should have a bit more customisation, but I really don't expect it to blow d2 out of the water.