Science

The two problems were in sustaining the reaction without blowing it up and then harnessing the tremendous amount of energy it produced. It sounds like they've managed to throw enough lasers into the chamber to sustain the reaction on a small enough scale that it didn't blow the facility up. They've always managed short bursts, but nothing sustained.

My best guess says water volatizes into steam a little too readily to be useful in a fusion reactor. They're going to have to figure out some other circulating substance to drive a generator, along with coming up with a tritium delivery system that will make it far less of a steamy waste of a scarce resource.

They're farther along than I thought they would be in my lifetime, actually.

Consider that a relatively small fusion reaction is enough to level an average city and most of its suburbs. Fusion is supposed to drive stars, not light bulbs. Getting it scaled down far enough is the part of the problem they seem to have solved