While I would initially dismiss this as it relies on the score difference instead of who won or lost, it really seems to elegantly solve a lot of problems I’ve been having with my implementation of the Points Rating System. This could be a case of the benefits outweighing the harms.
I love that this system provides for games with no handicap or incorrect handicap to be dealt with. Upon further reading, is this intended to be used with reverse komi and/or handicap stones at all?
How do you deal with the degenerate case where the stronger player knows they’re going to lose (after komi), but might still be able to come back if they can start a fighting somewhere? If only W/L matters, the stronger player will start the fighting and attempt to turn the game around, but with the above system, where the Difference in score directly impacting the change in rating, there may be a perverse incentive for the stronger player to just submit to the loss and work to make it as small as possible.
How do you deal with the other possible issue where one player is confident in victory (after komi)? If only W/L matters, that player may legitimately wish to favor safer moves, especially if they are the weaker player or they are very far ahead, but in the above system, both players will still be trying to maximize score? This is not so big an issue as the other mentioned case, and may even be desired behavior.
Another issue is what if one player rated 100 plays an even game with one rated 210? If the 210-player wins by 110 points, do they lose 9.5 * 0.2 == 1.9 ~= 2.0
points (because the score of the game for rating purposes cannot exceed 100.5 and the expected score difference is 110)? Or do you just disallow such matchups, perhaps requiring them to be played with some amount of handicap and/or reverse komi to bring the expected score difference below 100?
EDIT: another potential flaw with basing the system on score difference, being forced to play a losing game to the end, especially at high reverse komi. I have given very high (300+) reverse komi to TPK players, and if I don’t unsettle every one of their corners, the game becomes almost immediately resignable. If they were calm, protected their corner stones, they’d have at least a dozen points per corner (area scoring), so almost 50 points, which is more than enough to win a 300 komi game. At that point, I’d resign. But if the gap matters, then I would have to keep playing an utterly lost game to try and minimize the damage. (Note that it’s worse than just not killing the corners, I will be attaching to and “armpit hitting” their corner stones and assuming that anything which escapes to the center can just be killed later, so if it lives, there is no compensation whatsoever on the outside.)