What are the hazzards of warming food in the microwave without covering it?

Boom and Splat. Your food will explode, or dry out. Otherwise, shouldn't affect nutrition content significantly compared to covering. And for everyone, if your food explodes in the office micro, please clean it up. (you know who you are!).
None to health. If you leave it uncovered, if it overheats, you may splatter food over the inside of your oven. Best to get one of those plastic covers to put over the food when heating it. Microvwave warmed food is as safe to eat as the food you put in the oven.
Messy microwave? None really. Microwave works by rattling water molecules in your food which generates heat.