After watching the latest Avenger's tv spot , I couldn't help but wonder, if Thor's hammer is so heavy that the Avengers can't lift it, why does it not crush the coffee table it's placed on?
It's not the weight, but the Odin's enchantment which makes Thor's hammer unliftable by unworthy ones.
Remember, Hulk who is equally or more powerful than Thor tried to lift the hammer in Avengers movie (on Helicarrier), but failed.
In the Avengers: Age of Ultron movie, it has been revealed that
Mjolnir is actually lightweight. Thor told this to Vision. Low weight allows better swinging.