Why is platinum used as a catalyst in fuel cell electrodes (and elsewhere)?
Imagine that a fuel cell is like electrolysis - the other way around. 

In the electrolysis of water, you'd apply a certain potential (a voltage) to the electrodes and observe hydrogen and oxygen formed at the electrodes.

In theory, you need a particular potential to run the reaction. In reality - and this depends on phenomena on the surface of the electrode material - you often need more.

With platinum, however, this overpotential for hydrogen is zero. 

This means that you have no losses, neither in electrolysis, nor in the fuel cell.

This might be oversimplified but I hope it's not completely wrong ;)