Too Much - Japanese planting is sparse. Being British we can't help but pack plenty in, herbaceous borders are in our blood. Resist, resist, leave space.
Too Many - Britons are great collectors, we even refer to plantsman's gardens meaning 'they have one of every variety'. The japanese use a limited
plant palette, this immediately increases cohesion and tranquility whilst the repetition of form highlights the odd specimen plant.
Bamboo - Nothing signifies the East more to the Western mind, and we tend to overuse it. Bamboo does have a role in courtyard gardens, as either a sparse vertical or a low, grassy square, but not the overgrown clump.
Rock - We never seem to incorporate enough rock. I can see why though as selling a house with two ton boulders in challenges the buyer's makeover skills. However cobble and gravel zones will give punctuation to the plants.
Hard to overdo!