2d20 is miles ahead. Its controversial, because its ahead of its time.
But, this part of what Sly said, I don't agree.
All the story element is in the game is a dicing trigger. When the trigger happens, it's the green light to implement an effect.
This has been a part of roleplaying for a long, long time.
EGG talks about a DM interpreting dice rolls and describing effects in the AD&D DM's Guide of 40 years ago.
Lots of games have mechanics where "when this is rolled" (trigger), "then this happens" (effect).
With 2d20, the triigger is momentum and high success. "Hey, look what I rolled!" That's the trigger. It's the signal that you get to make up something different or extra about what your character is doing. What you make up is the effect.
That's no different than rolling a Critical Hit in 3E D&D (trigger) that causes extra damage (effect).
It's no different than several mechanics in the Mongoose Conan game, like an attacker rolling exactly the number of his foe's AC (trigger), as this indicates that the two have locked blades. The attacker is given a free sunder attempt on his foe's weapon, and if the weapon resists, the two are locked in a test of STR, pushing and pulling their swords against each other (effect).
There's nothing innovative about it. This design as been around a long time.
The only thing that I can think of that is maybe a little bit different is that the effect is not combat or skill oriented all the time. Story results are encouraged.
From my point of view, the Mechanic is not liberating at all. Rather, it's a straightjacket. It's more restrictive than, say, a standard D&D game (thinking of 3E, since that's what I play).
1. It trains the players to be creative only when the dice trigger it.
2. It can be seen as limiting the GM's control on the game--especially if the GM has to wait for the players to generate DOOM points before he can implement certain parts of his story.
3. In D&D, it is sometimes fun to go into a town and roleplay with the local merchant for supplies. Then, at other times, the ebb and flow of the game, as measured by the DM, dictates that dragging out yet another miscellaneous roleplay with a merchant would bore the players and slow the pace of the game, making it less interesting for the players. Dice have no way of measuring whether its time to have a cool story element or just dice-n-go. I've seen 2d20 GMs report how the dice indicate a big success but everyone at the table is fresh out of ideas but feel pressured by the dice to "come up with something". I've also heard about the same effect (the same story element) being used, over and over, when triggered by the 2d20 dice. The point is: Sometimes, in an rpg, you just want to dice it and keep on moving, and the 2d20 mechanics are not always in on that game plan.