People can access hacked clients too easily,thus making SMP pvp
hardly legit.
X-ray:
Hard to spot someone with it unless it's obvious tunneling,it makes mining pointless when you/others have x-ray to get straight to diamonds.
Radars:
Impossible to spot someone with them,making an unbush or hiding impossible.
Force Fields/Click aimbots:
Can be hard to spot,someone can just destroy you and you can't know whether or not they did it legit.
What I expected: People playing withouth hacks
What happened: Some people used hacks
How to reproduce:
1)Create a server
2)Look at the players for some days
Possible fix: Do not allow modded clients to join SMP servers
>inb4 invalid
Well I certainly wouldn't support a general ban on SMP client mods, I could however understand it being an option for individual servers. Even then though, I wouldn't like it for most servers, unless it were more granular (that is, configurably allowing/disallowing certain mods/mod types. Like the way flying was (is?). This however is likely to be very difficult to impossible until the official mod API comes out to regulate things a bit.
Also keep in mind that people can cheat the server in many ways not limited to mods as well, and that many (most?) people who use client mods in multiplayer do so within the rules of the given server. (Especially mods like Optifine.) Many servers would be completely impossible without client mods (Like Feed the beast servers.)