Re: Using d20 instead of 3d6 (DON'T KILL ME!)
I agree that the effect you've observed about modifiers was completely intentional. A difficult task is supposed to be meaningless to a master, difficult for the moderately skilled, and utterly disastrous for the novice.
But enough about that, as for how well it would actually work in practice, it seems possible to me. A roll of 10 is still the median, but there's no curve around it anymore. The range of the roll would expand some, now that you can roll 1,2,19, and 20. People would have to spend more points to make their skills really reliable, because of the greater range of the roll and lower predictability of having no curve, but that wouldn't break the system. If skills costs doubled with each level like GURPS, it would be a big difference, but skills have a flat cost in Hero, so it wouldn't matter that much.