I wont be the first to tell you gear is pretty much everything. The jump from tyrannical to grievous is one of the biggest ever. I know because my friends mage went from doing 45k ice lances when he dueled me to 80k when he geared up. Warriors are no exception.
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.