Common motherboard failure points?

So far I've had two or three cases where a PC motherboard just stopped working for no apparent reason. In one case, somebody jostled the system hard enough to disengage the CPU heat sink. In that case, I thought to myself "This is a slam dunk. Just replace the CPU." In all of those cases, the replacement CPU was cheap enough that I figured, what the heck, let's do it. I followed correct procedures (anti-static, thermal paste, etc.), but in none of these cases did the CPU replacement bring the system back to life.

Is there some component on PC motherboards, besides the CPU, that tends to fail more frequently than others? Something to test that might lead to bringing the boards back to life?

Thanks.