Guild Wars 2

Quick overview for those not familiar:
With the SC2 system, you gain and lose points for each win based on your rating vs opponents’ rating (similar to ELO style), but you get bonus pool points every day that will double your points received from winning (until the bonus pool is depleted). I think in SC2, the bonus pool is setup such that you only need to win a game every couple of days or so to take full advantage of the pool.
So in SC2, you have your ladder points (affected by win/loss/rating & bonus pool) and your hidden rating (affected only by win/loss/rating).

This would fix a couple of glaring issues with current system:
1. This would take care of decay. Decay would not be required as players that quit playing will be left behind by the people actually playing the game.
2. Players would feel like they are making progress. Their points for the season would continue to go up (from the bonus pool), even if they are not getting any better (win/loss hovering about 50% vs evenly matched teams).

This would not fix the matchmaking system though. That would still need to be addressed separately.

Something like this is a great idea.