One of the more interesting things to happen recently in the world of competitive gaming is that Magic: the Gathering gave up its ELO rating system for a new system called Planeswalker Points.
To recap, ELO is the system where you have a personal rating and that rating increases when you beat higher-rated players, and decreases when you lose to lower-rated players. It's effectively the same system WoW uses for Arena rating and competitive PvP.
Planeswalker Points are a purely cumulative point system. Every time you win a game, you get 3 points, with a multiplier for the type of event you are attending. More prestigious events have a higher multiplier. The big difference is that you cannot lose points, at least during a season. Points are reset to zero when a new season starts.
The reason Wizards of the Coast introduced Planeswalker Points was to encourage players, especially highly rated players, to play more. The thing about ELO is that once you achieve a certain cutoff rating, it becomes better to stop playing and sit on that rating for as long as you can.
For example, in WoW, let's say you needed a 2200 rating to get a new weapon. You've finish your 10th game and end up with 2201 rating. Are you going to play an another game? More often than not, you'll sit on that 2201 until you get the new weapon, even if it means not playing. You can't risk your rating dropping below 2200.
ELO also discourages players from taking risks, especially at more casual tournaments. A high-ranked player who beats a low-ranked players gains very few points, but if she loses, she stands to lose a lot of points. This makes life harder for lower-ranked players to climb the ladder, because they need to find and beat those higher-ranked players to gain more points. Often the top ratings become very static.
But if the Planeswalker Points never decrease, then--following Coriel's First Law of Skill--does that not mean we are measuring time, not skill?
The key here is that WotC controls the amount one can play. You can only play Friday Night Magic once per week. There are only so many Grand Prix. The difference between the amount of time spent by the average competitive player and the edge competitive player is not that large. And the multipliers reward winning at the more prestigious events. That makes attending smaller events more optional. Not to mention that there are no points for losing, only for winning. It is not like the old PvP system where the Grand Marshals simply played for hours on end.
One could also argue that the real metric is the ratio of earned Planeswalker points to total possible Planeswalker points at time T. And that ratio can decrease. Planeswalker points are biased towards players who play regularly, rather than a player who plays rarely, but always wins. But that is by design, a desired property of the system.
There are other differences between WoW and Magic. The biggest difference is that Magic never used ELO for match-making purposes. It was used solely to determine invites to high-end tournaments and as an overall ranking system. Match-making in Magic is done using the Swiss system on a per-tournament basis. For each round of a tournament, you are matched to an opponent of similar record. The 3-0 guys play each other, the 2-1 players play each other, and so forth.
There are disadvantages to the Swiss style of matching. For example, a poor player will often end up with a losing record all the time. They might go 1-5, 0-6, 2-4 in successive tournaments, dampening their enthusiasm for continuing to play in new tournaments. Whereas if a poor player is matched with someone of similar ELO rating, they can eventually expect a 50% win rate.
But there are disadvantages to ELO as well. Ratings can be manipulated, such that your rating, especially of an alt, does not reflect your real rating. But in Swiss or Planeswalker Points, your results are your rating, so manipulation is pointless. And as mentioned above, often the best way to maintain a good ELO rating is to simply stop playing.
Still, it's interesting to note the differences, and the different imperatives that push each system. Vanilla WoW swung way too far to the "playing for hours and hours" side of things, whereas modern WoW might swing too close to the "play only the minimum games" side of things. It would be an interesting exercise to design a system like Planeswalker Points for an MMO, given the constraints that people do not play in scheduled tournaments.