Some microcontroller indeed only require a bypass cap to operate. Others require some kind of external clock, like a crystal, resonator, R-C, or a digital signal fed in from elsewhere. Newer micros tend to have internal clocks, but these are usually only good to a few percent accuracy.
Boards with micros can be complicated because the system needs to do additional things external to the micro. For example, analog inputs may need to be scaled and conditioned before being presented to a A/D input. Micros are usually used to control stuff. That stuff can be complicated.
As for minimal parts list, see the datasheet. Most micros need ground, power (with bypass cap, of course), maybe external crystal or clock, maybe a filter cap for internal voltage regulator, possibly something to drive the reset input, usually some connections to get the program in, etc. For this information you have to read the datasheet, then also look at your overall system requirements. Those dictate how you'll use the micro, and therefore what else you need to connect to it.