On a survival server I took a long exploration walk and nightfall caught me in the middle of a desert. I decided to spend the night on top of a pillar (something I had never done before) and see what happened. I figured I could shoot mobs from my perch. I pillared up 11 blocks to get a good view of the surrounding terrain, but although I waited until daylight, no mobs appeared at all in any direction.
I'm just curious why this happened. Did I pillar up too high for my presence to register? Or could some mod on the server be the cause?
The mob cap is shared among all players in multiplayer, and while it is scaled with the number of chunks loaded by all players the mob cap can be taken up by mobs around other players (the way spawning works is that skylight halves the chance of a successful spawn attempt, even at night, so they are encouraged to spawn in completely dark areas). Another issue is that many servers lower the view distance and this causes mobs to fail to despawn in "lazy chunks" (chunks within 2 chunks of the edge of loaded chunks, which do not tick entities; they won't despawn even if you switch to Peaceful) if it is less than 10 since mobs despawn more than 128 blocks or 8 chunks way, so they need an additional 2 chunks loaded to ensure that they are ticked. This is particularly noticeable if you don't move since they will build up until the cap is reached.
Also, 11 blocks is low enough that mobs won't be able to spawn within a small area around you on the ground since they can only spawn outside of a sphere with a radius of 24 blocks, although they will still spawn further away (less than 24 blocks since you arn't right on the ground) unless you are more than 128 blocks above the ground, at which point they will immediately despawn (or not spawn at all).
There may be other reasons for a lack of surface spawns, although they should only reduce, not entirely eliminate spawning, such as a large cave system underground (though regional cave density is far less variable in 1.7 and later, and even in 1.6.4, or modded worlds with more/bigger caves, I see plenty of mobs spawning at night over the densest cave systems). You can use F3 to see if the entity count (the "E:" line, which is rendered/total entities, though it doesn't show entities that are too far away to be sent to the client, 80 blocks for most mobs, for example, there are only 51 entities here while MCEdit shows the proper number present. You'd also want to be able to switch to/from Peaceful to accurately count hostile mobs).