I thought I would share my ambivalent slaloming reactions to the D&D Next playtest documents now inexorably crawling across the net. Here's the first; it's a positive zig.
Plus or minus 3.325. On average.
That's the statistical impact of D&D Next's all-purpose mechanic to replace circumstance bonuses on d20 rolls: advantage (roll 2d20 and take the higher) and disadvantage (take the lower). But the impact across all possible chances is where this mechanic really shines.
The chart below plots out the effective bonus given by "advantage" across all possible chances to succeed against a given DC with a given bonus, from 1 in 20 to 19 in 20. I compare it against the flatter bonus given by the impossible but statistically equal +3.325.
At the same time, the advantage system doesn't favor long odds like the bonus system does, over on the left. And it makes it nearly impossible to fail when high skill and advantage coincide, with a final chance approaching 20 once added up, on the right.
It's styling, but most importantly, it's simple. One bonus fits all, is some good Old School mentality.