The current MAJOR problems with fusion boil down to two current show-stoppers: converting the energy generated to electricity and maintaining the reaction. The current primary method of energy conversion, heat to steam, is painfully inefficient and relies on absolutely massive, extremely precisely balanced chunks of metal moving at extremely high speeds. Other methods, such as those exhibited by solar panels, require some rather exotic and esoteric materials that don't take very well to being bathed in plasma, even magnetically contained plasma. Further, you have to have some way of maintaining the deuterium flow to the reactor. Too much deuterium in the reactor and you either get flares, with rather .... catastrophic results or the reaction is poisoned and never goes off. Neither are beneficial. And since we are required to contain the reaction to obtain the necessary heat and pressure required to initiate fusion, a magnetic bottle is required. Thus, the deuterium must either be passed through this magnetic bottle, with random and/or chaotic deflections to the particle stream, which would result in the deuterium not reaching the location required for the z-pinch or laser pinch, or the bottle must have a hole opened in it for brief periods and the deuterium pellet fired at extremely high rates of speed through said opening to minimize heat and pressure loss and damage to the containment vessel proper. Remember, we are talking about heat greater than that found in the envelopes of stars to make up for the lower pressure. You also have to remove the resultant helium, to prevent it from poisoning future reactions. Another long-term problem faced by fusion is that hydrogen fusion, even deuterium and tritium, is a rather energetic source of beta and gamma radiation. These emissions, along with the helium particles (alpha particles) will interact with the walls of the containment vessel, causing them to transmutate into their radioactive isotopes. Unfortunately, these isotopes tend to be the ones with short half-lives and thus high radiation levels. Further, the neutron bombardment inherent in any nuclear reaction tends to damage the walls on a molecular level.
Thus, while current experiments have nearly reached energy parity (same energy out as in), these are for single deuterium-tritium pellets that are precisely placed before the experiment. We've got a LONG way to go before fusion goes viable. There's a reason it's been 50 years away for the last 50 years...
P.s. Just to give you an idea of the power required to START a fusion reaction, recent experiemnts have required a pulse of 25 MW, for a total fusion reaction of ~.5 seconds and a power output of 16.1 MW.
