1. You are opened to all sorts of attacks, eg. lob, passing shot, dipping topspin etc. 2. On the other hand your options for attacking are limited. Unless you get that perfect high volley or overhead, you might find yourself having to hit up on the volley or even hit the half volley, the hardest shot in tennis. 3. A volley is essentially a block so its power and spin is limited, thats why even a perfect volley gets run down so often. 4. When your at the net, a minor mistake is fatal. When your at the baseline, a minor mistake is what it is, a minor mistake, not fatal.