Because memory foam is an open-celled material, the air can exchange between your body and the foam allowing you to sleep cooler.
Many manufacturers build air exchange layers into their mattresses to help cool even more, but I think this is not necessary except to help convince people that memory foam doesn't sleep hot.
Memory Foam Stinks
Again, this is a problem that may or may not be true.
Many of the mattresses made from memory foam have what's known as off-gassing problems.
Any memory foam could have a slight "new car smell" when you first receive it because its packaged in plastic.
Offensive odors are normally only found in foreign-made foam products that can use any number of toxic chemicals in their mattresses.
American-made foam is restricted by United States regulation from containing many of the harmful foreign-made chemicals, so they don't normally have any detectable odor.
Many people are concerned about the way memory foam mattresses are made to meet the new flammability standards imposed by the government.
These regulations are strict and require any mattresses sold in the U.S. to meet certain standards of flammability.
Many manufacturers of spring mattresses as well as foam mattresses may use boric acid or other harmful chemicals as their flame retardants.
Others use a variety of other types of retardants.
The different methods of flammability retarding can be used by manufacturers of any kind of mattresses so memory foam mattresses made with safer retardants are better than spring mattresses using boric acid or other harmful chemicals.
So those are the main 3 memory foam mattress problems that most people think are drawbacks to choosing memory foam but are really easily answered.