Here are some good/bad points about sand, comparing it to gravel:
sand looks better (its a fact

),
you cannot use an undergravel filter with it, the grains are too compact therefore there is not enough water flow between the grains,
can have a builup of toxic air pockets but this can be prevented/resolved by raking the sand every so often, corydoras catfish, kuhli loaches, some snails and others that rummage throught the substrate can also help prevent this from happening, also be sure to put no more than 2 inches of sand in the prevent this
fish waste sits on top, where with gravel it can fall through. This has advantages and disadvantages, it means that in a sense it is easier to clean because it sits on top, however poo sitting on top of sand doesnt look to good
Catfish, gobies, soles, spiny eels, and cichlids are all fish that love to rummage through the substrate, and sand makes it alot easier for them to do so, and coarse gravel can even damage some of these fish (cories barbels get worn down etc)
Sand is more reflective than gravel, and this can be a downside with some fish, who may show faded colours.
HTH
