Most games ration the top tiers of loot in some fashion. They do this to draw out the process of gearing up, and to give people an excuse to log in and continue to play.
For example, WoW uses raid lockouts combined with a random chance of items dropping. You can only do a boss once per week, and you have X% chance of your item dropping. This method is exciting, but can be streaky.
The percent chance can be for the entire group or for the individual, as in the case of WoW's personal loot system. I think the entire group method is better, as it is more apparent that the reward is for the entire group. As well, in a good group, very little loot is wasted. Unfortunately, as LFR proved, you can't trust a random group of strangers to distribute loot reasonably.
The other common method is an end-game currency where the amount you can earn per week is limited. Thus, you know an item costs exactly Y points, which will take you exactly Z runs. This method is perfectly deterministic, but rather boring.
Of all the rationing methods in the MMOs I've played, I like the system used by FFXIV's 24-man raids the best. A separate item drops for each 8-player group within the raid. If it's for your role, you may roll Need. Otherwise you can only Greed. But you can only win one piece from the instance per week. You can do the instance as often as you want. You can fish for a specific item or just take the first item for your class that drops. You can roll for your alternate specs if you want, but if a main-spec in the raid wants the item, they will win.
This "one item per week" restriction is a very blunt instrument. It's very meta as well. There's no real in-game rational for it at all. But it just works. People only roll on items they want, since there is a significant cost to winning. The tank gets the tank item if she needs it. There's no chance of a damage dealer winning the item over the tank. The item drops for the group as a whole, and people who win stuff get congratulated. It feels like a team working for a common reward, which is something that WoW's LFR has lost.
Of course, FFXIV also has an alternate raid currency. So even if you don't win anything that drops during a run, you accumulate the raid currency and can buy gear that way.