They swell up like balloons due to little air pockets trapped throughout their interior, but once those pockets pop you get a very saggy marshmallow (which returns to roughly normal size anyway once it cools.) Perhaps worst of all, you do not get the brown/black crunchy crust from roasting them over a fire. Faced with the choice, I quickly decided I preferred toasting them over gas burners rather than in the microwave (unfortunately, that often means cleaning marshmallow goo off your burners.)

Do not speak too quickly; around the same time I was discovering Properties and Effects of Microwaving Marshmallows, I also discovered ants will, if they come too close to a transformer, make a bee-line (or ant-line, not to be confused with an ant lion) straight for it. Apparently E-M waves act as a beacon similar to the little chemical trail they leave for each other, but the latter ensures that after the first ant dashes for the transformer, all the others will, too. That would be fine, except, of course, the high voltage quickly fries all their little insect bodies, incidentally shorting out the transformer.

This happened to our air conditioning compressor TWICE in a single summer. In Central Texas. It was not quite as bad as when we were without air conditioning and half the house had no power for a month after hurricane Alicia, but only because Austin was not nearly as humid as post-hurricane Houston (speaking of Houston humidity: Roaches also completely ignore microwaves, probably the source of claims they can survive nuclear war.) We still spent several weeks with no air conditioning during temperatures well into the triple digits. "How's the weather," indeed....

Since you asked, however, it is currently sunny here and temperatures are in the mid-seventies, easily the warmest it has been so far this year. In about an hour I will make my first trip to norsk kurs without either a jacket OR sweater. I still prefer cool overcast drizzle, but after several months of it, separated by half a year of blizzard, I have a new appreciation for warm and sunny.