The free-soling craze really took off when Peter Croft did Astroman in Yosemite over 20 years ago. The difficulty of that climb (5.11+) is only slightly less than the Regular Northwest Face route on Half Dome (5.12). However, Hannold upped the ante since the Half Dome climb is over twice the length of Croft's solo.
One of the most striking features of the Half Dome climb is the Thank God Ledge, where Hannold is shown with his back to the wall, contemplating his future. The ledge is 1800 feet off the ground, and narrows to under 1 foot in width with ZERO handholds along its length. If there's any wind that day, it's a harrowing experience even though you're "standing on a flat surface". A fall, when roped, would mean a serious pendulum swing, since it's virtually without any placements for protective gear along the ledge.
Only to be done by those with no families to leave behind.
Lee