All seems normal and healthy until a player tries to connect...
2011-08-29 14:15:47 [INFO] papand13 [/192.168.1.102:56960] logged in with entity
id 112 at ([world2] 51.70216561567827, 69.0, 195.90846125401683)
2011-08-29 14:15:47 [SEVERE] java.lang.NullPointerException
2011-08-29 14:15:47 [SEVERE] at java.lang.System.arraycopy(Native Method)
2011-08-29 14:15:47 [SEVERE] at java.io.BufferedOutputStream.write(Unknown Sourc
e)
2011-08-29 14:15:47 [SEVERE] at java.io.DataOutputStream.write(Unknown Source)
2011-08-29 14:15:47 [SEVERE] at net.minecraft.server.NetworkManager.d(NetworkMan
ager.java:263)
2011-08-29 14:15:47 [SEVERE] at net.minecraft.server.Packet.a(Packet.java:116)
2011-08-29 14:15:47 [SEVERE] at net.minecraft.server.NetworkWriterThread.run(Sou
rceFile:107)
2011-08-29 14:15:47 [INFO] Connection reset
2011-08-29 14:15:47 [SEVERE] at net.minecraft.server.Packet51MapChunk.a(Packet51
MapChunk.java:88)
2011-08-29 14:15:47 [SEVERE] at net.minecraft.server.NetworkManager.f(NetworkMan
ager.java:107)
2011-08-29 14:15:47 [INFO] papand13 lost connection: disconnect.genericReason
2011-08-29 14:15:47 [INFO] Player not found.
2011-08-29 14:15:47 [INFO] Player not found.
2011-08-29 14:15:47 [INFO] Player not found.
Then all hell breaks loose.
Try disabling your plugins and seeing whether players can get on (afraid this means water will spill out of the creative gates, but you can clean up later). If that works, bring the plugins back one by one. Sometimes 2 plugins interfere with other. They work fine seperately, but have both on and it causes it all to crash out.