To understand how difficult this was, one must first understand climbing grades.
The YDS (Yosemite Decimal Scale) is generally what is used in the US to grad climbs. It ranges from 5.0 to 5.15, at the moment.
A 5.0 climb would be a scary 'hill' climb that might involve a couple technical moves and a lot of scrambling up steep terrain.
At about 5.4 to 5.5, average joe hiker will feel very uncomfortable and would be wishing for a rope, protection on the wall, and a good belayer.
Now, I believe (I have been rock climbing for about 3 years) that most people, that are generally pretty fit and have the ambition to stick with rock climbing, can climb up to 5.10 relatively easily. That is not to say that 5.10 is not difficult, it requires much practice and strength/endurance building.
At 5.10 the YDS will start with minor grading indicated by A, B, C, D. If you start thinking about getting into 5.12 range, you are a pretty serious climber and very strong. Not too many people can climb 5.12 consistently. To compare this to running, I would say it is equivalent to breaking 15:00 in a 5k, so about .5% to 2% of climbers can climb 5.12 outside on rock. I understand that 2% of runners cannot break 15:00 in a 5k, but there are far fewer rock climbers than runners.
The most difficult pitch (generally one rope length of climbing, 30 to 70 meters) on the Dawn Wall is 5.14d. So to put this in 5k terms, you are getting pretty close to the world record, I believe the hardest pitch recorded is 5.15b, there are 5 to 10 guys right now that can climb 5.15. Now this 5.15b pitch was done off the ground as a single pitch. What we are talking about on the Dawn Wall is 17 pitches ranging from 5.12b to 5.14d, with several pitches of 5.14 climbing.
This is a pretty impressive feat, however it will be repeated soon and we will start seeing more climbers 'free climbing' long hard routes around the world. Tommy and Keven are on the forefront of some pretty historic climbing.