Diesel fuel isn't flammable.
Diesel fuel isn't flammable. You can pour it out into a puddle and throw a match on it and nothing will happen but the match going out. It's only flammable when put under high pressure, which is how a diesel engine works. Gasoline, on the other hand..
Hydrogen doesn't burn in the sense that other fuels do. As John points out, it flashes as the H2 combines with other elements in the atmosphere and gives off energy in the process. The biggest advantage to fuel cell vehicles is that they're much lighter weight per kWh than equivalent batteries of any chemistry and have a much longer lifespan than do even the best of batteries for automotive use. Fuel cells also don't have a lot of toxic or potentially toxic materials in them and are relatively easy to recycle in comparison to most battery chemistry. Besides, a 20 minute fillup for 300+ miles of service versus a 4+ hour fillup has obvious advantages.
The reason every major auto manufacturer is fiddling with fuel cells and the reason Europe is so big on them is that it's potentially a much more viable automotive fuel source than are battery electrics. Especially in larger, heavier vehicles. Hydrogen also has the advantage in that much of the existing fueling and storage infrastructure can be used or adapted to use for it.