Halliday & Matthiessen (1999: 186):
Qualities tend to be experientially simple, specifying values along a single dimension or scale such as age, size, weight, loudness, colour, according to either scalar or binary distinctions (e.g. scalar: 'large' — 'small', 'tall' — 'short'; binary: 'male' — 'female', 'dead — 'alive'). Things, on the other hand, tend to be experientially more complex than qualities. They are often definable in terms of an elaborate taxonomy where several dimensions (parameters) are needed to distinguish them.
Consider for example the scale of size. The various qualities named by large, big, giant, small, tiny, and so on indicate a region on the scale, but do not specify the "substance" of whatever it is that size is being ascribed to. If we look for objects that are characterised in terms of size, we will find e.g. a giant, a morsel, and a mini. These, however, involve far more than the single dimension of size. A giant is 'any imaginary being of human form but of superhuman size and strength'. A morsel is 'a small bite or portion of food'. A mini is usually understood as 'a small car capable of holding a normal complement of passengers'. Small objects are typically objects of some particular kind, e.g. droplet, booklet, and kitchenette.