My apologies to the Mythbusters for borrowing their motto as title of this post. After over 1,000 battles in World of Tanks I have a win:loss ratio of very close to 50:50, with over 500 battles each won and lost, and less than 50 ending in a draw. And everybody else I've talked to has similar numbers, World of Tanks has a very balanced matchmaking system. While players tend to complain about any matchmaking algorithm that doesn't let them win all the time, I do think that a 50:50 win:loss ratio is ideal. Not only for reasons of fairness, which are debatable, but also because I believe that if you have a 50:50 chance of winning or losing, your personal contribution to the outcome is most pronounced. In assured victories or losses it matters not so much how you play.
Moving from World of Tanks to World of Warcraft and other MMORPGs, I have to observe that 50:50 chances in MMORPGs are rare in PvE, and not always given in PvP. Furthermore in PvE there is a bizarre switch in win:loss ratio when you reach raiding: While leveling up your chance to die in combat is very close to zero. On a raid night your chance to die in combat is very close to 100 percent. And even if you define winning and losing differently, a typical progression raid on a new boss is far from a 50:50 chance of succeeding. It isn't unusual for a guild to have certain bosses "on farm", with a high chance to win a combat against that boss, followed by the current "obstacle" boss on which the guild tends to wipe a lot.
Personally I find 50:50 chances to be more fun. In World of Tanks you never know what you will get when you press the Battle button. And even once you are inside a match and see what you are paired against, it isn't all that obvious to predict the outcome of a match. Sure, part of that unpredictability is based on luck. In one battle you decide to sneak around the left side and manage to destroy the enemies' artillery to great success, in the next battle the same tactic ends with you running into three enemy tanks guarding that flank and obliterating you. And there is always luck involved in how competent the other 29 players on the battlefield are. Nevertheless World of Tanks very much gives me the impression that my decisions matter for the outcome of the battle. If I observe what the rest of my team is doing, and what I can see from the enemy, I can make an intelligent tactical decision about what to do, and influence the outcome of the battle. That wouldn't be the case if victory or defeat was assured from the start, like it is in so many PvE fights in MMORPGs.
I do believe that MMORPGs would be a lot more exciting if most combat had a 50:50 win:loss ratio. That is obviously harder to implement in PvE than in PvP. In PvP you just need to pair people of similar abilities and strength against each other, while PvE suffers from the pervasive idea that the challenge has to be the same for everybody; that ends you up with the problem of people having different degrees of skill, so a perfectly balanced 50:50 win:loss PvE mob in fact has the skilled players still win all of the time, and the less skilled lose all of the time. But I think that problem could be overcome by giving people a better choice against what difficulty level of mobs to fight, and getting the incentives right for opting for the greater challenge instead of the easy farming. Too bad I haven't seen that in a MMORPG yet.