However, your frightening and my frightening are completely different.
IMO this is less subjective than you're thinking, and the WP definition is not very good.
The issue is not the spacing of the protection per se. The issue is whether or not you really have a meaningful belay, which can depend on the spacing of the pro. Here's an example of a 5.8R slab climb that I've followed on: http://www.mountainproject.com/v/surprise/105788042 . I'm going to guess some distances from memory. The start of the first pitch was a broad ledge. From there I belayed the leader up the slab maybe 10 meters to where he clipped a bolt, and then another 20 meters to the end of the pitch.
As with any lead where you start on the ground, he had no effective belay until he reached his first pro. Because this height was large, the consequences of a fall would have been bad. Basically a fall would not have been acceptable here, and he had to be really sure that he could climb 5.8 slab with no significant chance of falling. This is different from a climb where you can get your first pro in very early -- on that type of climb, it can be acceptable to fall.
Once he reached the bolt, 10 meters up, he had to continue up another 20 meters without further pro (assuming for the sake of argument that my memory is right). For the first 7 or 8 meters above the bolt, I was able to give him an effective belay. Past that point, there was enough rope out so that if he had fallen, he would have hit the ground. Once he was 10 meters above the bolt, he would have hit the ground without even stretching the rope. So again, this was a situation where it was not acceptable to fall.
So to me, this is the relatively objective definition of an R-rated climb. You have at least some points in the climb where you effectively don't have a belay. During those portions of the climb, it's not OK to fall.