This is just my educated guess, but I think Macs tend to run hotter than a lot of other laptops because Apple works so hard to engineer their products to be as thin as possible. This means everything is packed inside the case super tight. . I think it's that because everything is so tight inside, the heat isn't as easily channeled out of the case, so things run a little hotter inside the case. Remember that Macs are essentially made of the same components as Windows machines.
That being said, I don't think it's really anything to worry about. What's hot to a person isn't hot to a computer. Of all the people who report OVERHEATING!!!!! I have not seen one case with the current builds where the computer is actually overheating. All the people who've been freaking out have had computers that are running within specs.