1) I do not understand the hype over the hydrogen economy.
Given that every time you convert energy from one form to another you lose energy, why introduce another conversion? The best large scale way I know to manufacture hydrogen is with electricity (electrolysis of water or whatever). Dunno the efficiency of this process, but where's the electricity come from? Existing generation technologies such as solar, hydro, wind, and thermal (fossil fuel) plants. The only big advantage I see in this is that a greater percentage of fossil fuels get burned at point sources, where emissions (SOX, NOX, HCs, etc) are easier to control. But it does nothing for CO2 emissions.
2) Gasoline Transportation.
Currently we have crude oil getting pumped, shipped, refined, and burned in cars. Gasoline is a very compact and convenient energy supply for transportation, and it requires no energy conversions to be used (only refinement). The major problems with gasoline are two-fold: (a) its hard to control emissions in such a non-point source as millions of cars, and (b) existing cars are very inefficient.
We've already solved (a) in large part - modern cars burn pretty clean. (b) is an emerging (emerged, really) technology, and if the market would just force people to consider fuel efficiency when buying their SUVs without hybrid drive trains, then fuel economy would... double?
3) Natural Gas.
Natural gas is the best fossil fuel there is from a greenhouse perspective - it has the lowest CO2 output per unit energy released. 1 carbon per four hydrogens, longer hydrocarbon chains approach a ratio of 1:2. (hmm. would molecules like acetylene with double or triple carbon bonds be an exception to this rule?)
Natural gas is great for heating purposes, because it is easy to achieve very efficient utilization. Common technologies such as residential boilers and furnaces can easily hit 85% efficiency, and larger or fancier systems can approach 98%. To the extent that our society is going to use natural gas as a fuel, the perferred use should be for heating because heatings is such an excellent use of this resource.
4) Electrical Generation
4.1 Thermal plants
Thermal plants are pretty medicore. The old rule of thumb for thermal plants is of the chemical energy (fuel) you put in, you get 30% out as electrical energy, 30% out as useable heat from the coolant system (like a car radiator), 30% goes out the exhaust stack, and 10% goes to miscellaneous losses. In the worst case, society gets 30% of the energy as electricity and the balance is dumped to the environment. Newer combined cycle plants (CC plants) use the exhaust gases to drive a secondary turbine - these plants can get up to 60% of the energy out as electricity, but they don't govern loads well, so they need to be run at constant output and thus are only suitable for producing a (large) fraction of the total energy demand. Some thermal plants are situated near other facilities so the coolant system can be used to heat adjacent buildings. Given that people don't like to be right next to smoke stacks and district heating involves a lot of pipes and stuff, the application for this is somewhat limited. But where it works, its great. Doing this with a CC plant and you might be approaching 80 or 90%. (?)
4.2 Hydro plants
Hydro plants are terrific in that they can convert the potential energy of water to electricity with something approaching 80-90% efficiency, which is hard to beat. Hydro is very site specific, and when not done right has bad environmental consequences. However, when done right, hydro is very environmentally friendly, and once the construction costs are paid off, hydro is the cheapest electricity source known to man (<1 cent/kWh is not uncommon).
4.3 Wind Plants
Wind is finally becoming economical. Wind has a role in the electrical grid, but it is a limited one. Wind energy is erratic, and is difficult to integrate with other generation technologies. Its not too hard to tie wind in as a small power supply on a big grid (say up to 10%) but as the percentage of wind generation increases, integrating wind with other generation sources and maintaining acceptable frequency and voltage in the system becomes very difficult.
4.4 Solar Farms
Far as I know, economics aren't there yet.
4.5 Tidal or wave energy
Haven't seen a great system implemented on a big scale yet. Would like to.