"There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult."
Found a post over on the 37signals site where Jason F. lists 'Quesions I ask when reviewing a design'. Might come in handy for some upcoming UI work we've got planned :)