Monks are amazingly good at 1v1 and 2v2 like I said. There's someone who does only 1v2's as a monk and hes like 1800 in 2s I think. They have amazing self-sustainability and mobility. That doesn't translate so well in 3s though. One of the comps I play is mage/ele/hpally and our teams mage and ele kill monks pretty easily. All they really have is karma and against magic diffuse. After that they are squisher than rogues. Self-healing doesn't do much against 2 dps.
Monks are actually a very good class, but they require a high level of skill to play effectively vs. say, a warrior or hunter. Thats why you don't see too many, that and they are a new class.