[Mippo]

On a very basic level, there are three main reasons people continue playing MMORPG's.

1) Achievement
2) Competition
3) Social

Almost everyone who plays falls into 1 of the 3 categories, some people falling into multiple categories. Without explaining it in detail, the #1 reason people continue playing MMORPG's is achievement. People like accomplishing things, it makes them feel good. This directly correlates to a risk vs reward system and hence why it is the #1 most important thing you have to get right in your MMORPG.

People expect that for time spent playing, they will be rewarded. In most cases the EXPECTATION of a future reward is what keeps people playing. A game designed correctly is properly rewarding players for their time. In most games this means itemization is one of the most important aspects of the game design as the power of the items dictates the value of the reward, a key component of the risk vs reward equation. If the items suck, like they did in AOC, the risk vs reward formula is now broken. People can accomplish what they set out to accomplish, but they aren't being adequately rewarded for their time. The same was true in Warhammer, where there was little correlation between time spent and rewards obtained. In fact, the system was skewed in the wrong direction. Renown Rank 80 took a ridiculously long time to accomplish but provided very little reward over what people already had. That doesn't fulfill the Risk vs Reward formula. The harder and longer something takes to achieve, the rewards should be greater in proportion.

WoW is one of the only games that is setup to consistently reward players for time spent playing. Even if you lose in the battlegrounds, you can still measure the amount of time needed to obtain the next item you want. The power of the item, and the time spent obtaining it is an important factor in Risk vs Reward. The proverbial carrot keeps people playing as the goals are obtainable for all types of players.