The number of mobs is not properly controlled by each player. Thus, if a player builds a dark room and is on for a while, they will get all of the mobs. If a second player then joins and is near a dark room, the mob cap doubles. This, the first player gets 1.5 times the normal mobs, and the second player gets .5 times the normal. The fact that another player controls the amount of mobs and the difficulty for a distant, separate player is clearly a bug.

Real bug report format:
What I expected to happen was...:
When a player logs in, the average amount of mobs that spawn near them is the same as the first player that spawns.
What actually happens is...:
The second player gets half the mobs of the first, the third 1/3 the amound of the second, ect.

Steps to Reproduce:

1. Create a superflat world with some preset that has controlled amounts of mob spawning, eg "7,10x0,1,32x0,2x7"
2. Run that world in mp.
3. Join and teleport to "1000,46,1000".
4. Wait for about 5 minuites until the mobcap is reached (e in f3)
5. Have another player join, and teleport to "-1000,46,-1000".
6. Wait 5 minuites.
7. Note how the amount of mobs by player 1 is about 1.5 times the amount at player 2. How does this make sense?

Watch: http://www.youtube.com/watch?v=q_DwPJZd2BU