It all comes back to one principle: everything is a battery. Any source of energy that we have is essentially just energy from the sun repackaged. Solar is obvious, but even biofuels, wind, oil and coal are (oil and coal are from plants, and wind is from pressure differences from heating, although some is probably from the earth's rotation). The only exceptions to this rule are geothermal and nuclear, (and geothermal is really nuclear, radioactive decay is what heats the earth), but neither of those can power the world.
Why does this matter? I think it matters because when you realize that no matter what we do, we're taking energy from the sun and storing it a battery of some form, certain types of fuels just stop making sense. Hydrogen has so many problems beyond this it hardly merits mentioning, but it's also the cleanest parallel. Hydrogen is a really bad battery, it stores about 40% of energy that comes into it (meaning if you use 100 J to separate water into hydrogen and oxygen, then burn the hydrogen to get water and energy, you'll get about 40 J back) (you might wonder where these numbers come from, which is a presentation I saw last year. I can't find them online, take them with a grain of salt. The point still stands that electrolysis will always be less efficient than a simple battery for energy returns, some other way of generating hydrogen may be better than electrolysis, but then all the other problems with it remain). Considering that current batteries are much, much better than that (and getting better), hydrogen just doesn't make any sense. Even more so when you consider that we'd still need to get energy from somewhere to separate the hydrogen from something (currently it comes from natural gas), and it's not an energy-generator, just a battery.
Biofuels are just as stupid. We're using plants as solar-cells, then chemically modifying them into batteries. Why not just use solar cells and batteries? Those can be placed anywhere, and we don't have take precious food out of the world's farms for fuel (which, by the way, is part of the reason food prices are at all-time highs). Biofuels may be promising in some regards, but I don't think we'll ever be able to generate appreciable amounts of fuel with them, simply because we'll run out of arable land, even if we turn the entire earth into a large farm (which would be beyond stupid).
My simple point is this: all of our energy ultimately comes from the sun, so we should collect it directly and store it in the best possible medium. We should still research and consider other methods, but I'm convinced that none of them will prove tenable.