Size - that's about it. Good for installing in smaller build PC's, but no less powerfull than full size boards. Drawback is you will have less slots available for expansion of additional accessories/peripherals.
Size is the advantage.MicroATX is usually much smaller than normal ATX and usually has fewer expansion slots like PCI (+variants), fewer DIMM sockets (usually 1 maybe 2) they are often built around...
ATX improves upon AT by making adding and removing components easier, providing greater support for I/O devices and processor technology, and lowering cost.