I'm absolutely sure I am not in peaceful mode I'll even provide my server properties at the end of this. I've tried switching between hard mode to peaceful and back again, nothing works. I've waited a few night, with no torches and far away from spawn points, no mobs spawn, not even in the caves. This is not a bukkit server and there are no mods or plugins. This has been happening since the recent 1.7.2 update and it's only in multiplayer server. If I play a in single player they'll spawn but if I transfer the world over to multiplayer all the mobs disappear. I can spawn mobs with a spawn egg but once you kill them they're gone. I've tried several different solutions from different forums but nothing helps.
#Minecraft server properties
#Wed Nov 20 10:50:50 PST 2013
generator-settings=
op-permission-level=4
allow-nether=true
level-name=world
enable-query=false
allow-flight=false
announce-player-achievements=true
server-port=25565
level-type=DEFAULT
enable-rcon=false
force-gamemode=false
level-seed=
server-ip=
max-build-height=256
spawn-npcs=true
white-list=false
spawn-animals=true
snooper-enabled=true
hardcore=false
online-mode=true
resource-pack=
pvp=true
difficulty=1
enable-command-block=false
player-idle-timeout=0
gamemode=0
max-players=5
spawn-monsters=true
view-distance=10
generate-structures=true
spawn-protection=1
motd=A Minecraft Server
Best Answer
What i would suggest is downloading the server package again, and copying the new properties file into the folder where your Minecraft server is located. If you have edited anything in the file (which it seems you have) try running it stock settings and see if that works.
Then, change settings one at a time to ensure that you can see what setting is causing the issue.
I've run a Minecraft server on Windows and Linux, and never encountered this problem, but hopefully this solves yours.