The client is open source, which is a good place for hackers (or modders, hacking is such a harsh term). It would be easy to double or triple what exp they get, walk faster get more more money from selling, I haven't looked to far in there, but it shouldn't be that hard

Also they could make it so they can attack faster, hit harder and do it while walking.
Also have it check the users current process list and make sure there isn't anything running that could harm TMW in any way (Such as packet sniffers/editors). Check to make sure all values going in is a value your letting them have.
Thats about all I can tell you now, I will look further into this later, Thanks for your time.