It's my understanding that the sort of fusion that goes on in the sun takes four hydrogen (H-1, one proton and one electron) atoms and crunches them down into a single helium atom. Not knowing a great deal about fusion mechanics, this both confuses and intrigues me, since the process is taking four protons, and turning two of them into neutrons, somehow. Adding up the atomic mass of four hydrogen atoms and comparing the result to the mass of a helium atom tells me that about 0.71% of the mass of the hydrogen is being lost; which, while not particularly efficient for a nuclear reaction, would still be enough to meet Earth's annual energy demands with less than 200 tons of non-isotopic hydrogen.
This, obviously, is too good to be true. Current fusion reactor designs that use rare isotopes of hydrogen or helium already require more energy to use than they produce, and THIS concept isn't even being considered, apparently. So before I go about meeting my sci-fi civilization's insane energy requirements with fusion reactors that consume as fuel the second most abundant substance in the universe (the first is stupidity), just how difficult would it be?
TL;DR, what are the problems with creating a fusion reactor that runs off of non-isotopic hydrogen, and are those problems theoretically solvable for a sufficiently-advanced civilization?