In d20 (and most versions of D&D) the base roll is a d20 against a 10. Each number on a d20 can be expressed as 5% of a whole. Needing to roll a 10 or higher translates to 55% chance of success.
In World of Darkness the base roll might be a dice pool of four d10s against an average Defense of 2, which lowers the pool to 2d10. To get one success then would translate to 51% chance of success.
There is no average in GURPS since it's a point-buy system. Assuming a character with a DX of 10 and no combat skills, you can brawl without a skill but the character doesn't do as much damage, and the percentage chance to hit is 50%. But in GURPS your opponent can dodge the attack, and assuming a similarly unskilled character with a Dodge of 8, they have a 26% chance of evading that attack. Which means that an average character only has a 38% chance of successfully hitting another average character.
In Deadlands... forget it. It's too chaotic.
I've been tinkering a lot with 2d6 and 3d6 lately so just for fun, if you assume you need to roll 8 or higher on 2d6 the chance of success is 41%.
Extreme results have differing percentages as well. Assume you need to roll the highest number possible, and on a d20 it's a 5% chance, on 2d10 it's 1%, and on 3d6 it's less than 1%. On 2d6 it's 2.78%
Which then begs the question: Why don't people judge their games by the percentages? The more I crunch the numbers the more attractive 2d6 becomes to me. Yet most games, and probably most gamers too, shy away from using 2d6 because it's too plain or generic. There's a combat system waiting to be born out of 2d6 that nobody has adequately explored yet, or maybe they have and I've just never seen it.