Once you block one or two ears more or less with a headphone, talking becomes uncomfortable. Most people don't realize it but every "normal" phone in the world blends in a fraction of the mic-signal of your own voice into the ear-signal to bypass this discomfort.
Although most people block two ears while using Skype with headphones, Skype does not support the "blending feature", which is a serious and very unprofessional shortcoming. Two other sound-enhancing features it lacks are compression, and added fake-noise to overcome the drawbacks of squelching. But this is to much tech-talk for now. Skype should hire a telecom engineer to enhance things like this.