There are three big problems,
1) Generating the energy source
2) Transmitting or transporting it to the end user
3) Efficiently turning it into work, while not generating any more pollution than necessary.
Re #2 - the transport media is the key to adoption. Gasoline and diesel are relatively easy because they are stable, they pack a fairly big power load per kg, they are reasonably safely transportable, and we have already invested in the distribution network. Biodiesel and Ethanol based fuels are easiest to implement because they can piggyback on much of the existing infrastructure.
Propane and natural gas are cleaner than oil and gasoline, but are a notch up the scale in transport and distribution problems. Hydrogen burns clean, but transport and distribution are real challenges.
Re: #1 - It's not quite accurate to say that if you burn oil or coal to make hydrogen (or electricity or ethanol or whatever), you are no further ahead than burning that oil or coal in a vehicle or home. A large generating / cracking plant could be quite a bit more efficient and less polluting than 1000 individual homes or vehicles because of efficiencies of scale. A large scale burner or fuel cell plant can benefit from technology too large or too expensive for individual use.
Re: #3 The end user efficiency and cleanliness have to be taken into account. Pure electric vehicles are not yet practical, partially because the cost and resource/disposal problems of the batteries are not solved. Fuel cell vehicles have to solve the challenges of fuel storage.. Fuel cell 'batteries' have the problem of waste heat disposal.
Nuclear vehicles are non-sensical for this reason - the powerplant would be large, heavy and unsafe (not to mention the distribution method!) But Fission generation may be practical in large generating stations.