Yeah, double damage for rolling a 20 doesn't really count as a true "Critical Hit" mechanic. Same name to the concept, but the result is very different between them and RM.
We actually went with rolling a 20 was x2 damage and rolling another 20 after that was insta-kill. Then we found the first Arms (Claw) and Spell Law books (and integrated parts/concepts from RM into D&D at that point).
I think the D&D we played (2nd Ed) was a good system, it just became too simplistic for us... which many people LIKE, so I'm not knocking it, it just wasn't out cup of tea. As I always said to people: Wouldn't you like that Fireball your 15th level Wizard just cast to, instead of merely cause 10d6 damage, do 42 points of damage, set your foes shield on fire, and cause them to be stunned for a round?