In SMP, the way mobs interact is different from Singleplayer. In Singleplayer, if you spawn 100 spiders (as an example) they would all fit in a 1x2x2 space. In SMP, if you tried to cram all of them into that same space, they would glitch all over the place, and you would see them teleporting out, even if they're in an enclosed area. So, none of the exp farms shown on youtube would be very efficient in multiplayer. Also, the lag from the server would decrease the efficiency of the traps/towers, because rather than a mob spawning every ~25 seconds from a spawner, it would take maybe 2-3x as long. If you do the math, waiting for mobs to spawn, then attempting to kill then while they glitch everywhere, is not going to be as effective as building a house, or mining sand as Fettuccini suggested. Hope this helps!