Originally Posted by meanwhile

They have poor venting, so your head gets extra hot and sweaty. Plus most cyclists know nothing about helmet effectiveness - they assume that everything in the store works equally well. Many of them assume that a helmet will work if you get hit by a car - which is a pretty odd thing to think of a few hundred grams of packing foam.

Other than looking for a good shell, look for a helmet that fits you well and has a good retention system - one that will keep the helmet snugly in place and level without being uncomfortable.