Seeing through walls is pretty useless in GW but how about hacks which alter character and skill effect rendering? Monks would be rendered with blue transparency, warriors with yellow transparency and so. Spell effects would have some noticeable color code effects so for example mesmers can see them easier and interrupt. The bad side about this is that people who know what they are doing, can manipulate a game specific rendering profile so video card will render certain things very different only in that certain game. This is completely invisible for server detection. Nothing of this will go servers because it is client side process.
My question is that what Anet can do about these? I'm pretty sure, some guilds might use them. Perhaps not top guilds because they have proven their skills in monitored sessions.