Wrong kind of fuel cell. The fuel cell being referred to here is more than a gas tank. It is a device that can convert a gas (usually hydrogen) directly into electricity. Some of them work by stripping the electron off the hydrogen atom sending it through an electrical circuit to do work (drive a car) and recombining it on the other side. The advantages are supposed to be low or zero emissions (water is supposed to be the only by-product) and a more efficient way to produce energy. It is also supposed to help reduce our dependency on foreign oil.

Some of the problems I see are:
1. Bulk: It is very difficult to get the fuel cell small enough yet still produce enough energy to run a car.
2. Storage: Hydrogen atoms are so small there is no tank that can hold them long term. Think of a latex helium balloon. In a couple of days the helium has leaked out and the balloon is lying on the floor.
3. Cost: Fuel cells are expensive to produce.
4. Oil dependance: Doesn't really free us from foreign oil. The best source of hydrogen on today is crude oil. The reason oil and gasoline are called HYDRO-carbons is they have hydrogen atoms in the molecules. So the only way it would work to free us from foreign oil is if it was a much more efficient energy producer. Kind of like the Japanese mini-cars of the late 70s and 80s did. That worked really well for us today right? (sarcasm)
5. Safety: Most fuel cells that I'm aware of run at very high temperatures to produce the reaction. In the case of an accident things could get very out of control, very quickly.

Most of these problems are not insurmountable and many have already been addressed. Just don't believe that someone will throw a switch, fuel cell vehicles start rolling and all our energy problems will be solved.