I read a while ago that nutmeg and cinnamon are really similar and you can easily substitute one for the other. Yet many recipes call for both cinnamon and nutmeg so it seems like there must be some kind of difference.  What are the specific differences between cinnamon and nutmeg? And are they significant enough to justify buying both cinnamon and nutmeg? 
They're completely different.    Cinnamon is warm, woody, hot, sweet, spicy, bark.  Nutmeg is eggnog. If you want something to taste like Christmas, use it.    They're both delicious, versatile, and can work well together.    By volume, you can use much more cinnamon than nutmeg.  Nutmeg is "a dash of" kind of spice, while cinnamon can be mixed with butter or sugar and slathered on basically any pastry.  (Equivalent amounts of nutmeg would probably get you stoned, but that's for another forum).  Also, nutmeg shares flavor notes and aroma with allspice and cloves. 