9:23 p.m. on January 8, 2012 (EST)
All good points. Having been a climber for most of my life, rating a climb is, as mentioned, sometimes a personal experience. There were climbs in Yosemite that I had a very difficult time getting up, only to flash them on another try. It really does depend on how you are feeling on any particular day. There is one climb there rated 5.10b that I've NEVER been able to climb. It's just not my style and requires a lot of upper body strength.
Also, climbs rated harder than 5.9 (using the YDS) are broken down into sub-groups using a, b, c, or d. An "easy" 5.11 would be rated 5.11a, and a hard one at 5.11d. And trust me, there is a big difference between a 511.a and a 5.11d. I was rarely able to even follow a 5.11d, let alone lead one.
Also, as mentioned, a climb is rated by its hardest move. The very first 5.11 in USA was put up at Suicide Rock, near Tahquitz. It is several pitches long, but only has a couple of 5.11 moves on it. The rest is 5.10 or easier and has great resting stances. It's a great route, worthy of the 5.11 rating. I know, I've led it.
But there are also routes rated harder than any actual move on it. Reed's Pinnacle in Yosemite is a classic example. It's rated 5.9, but has no single move harder than 5.7 (I think - it's been a long time since I've climbed it). But the route is a sustained 5.7 with no real resting places, so it got a harder rating.
Bill makes another good point stating that different types of climbs (face, crack, off-width, etc) rated the same "feel" different. One might require more upper body strength, the other a totally different technique. You may be good at one, bad at the other.
Still, other locals rate differently. A 5.9 in Joshua Tree is a lot harder than a 5.9 in Yosemite. When I used to climb there, I down-rated by ability one grade, just to be safe.
I don't agree that routes are rated harder than they really are just to pad the climb so someone thinks, or thinks they feel like, they are climbing harder than they are. The climbing community is a strange bunch. If a route goes up and is rated 5.11c and subsequent parties think it's more like 5.11a, that climb will be down-rated. Son of Sam in Yosemite is an example here. For the first couple of years after its first ascent, it was rated 5.10a. Later climbers thought it was rated too hard and it was down-rated to 5.9.
As mentioned, gym rats might be able to climb 5.11 inside, but rarely can they climb that hard on rock. They haven't learned to place protection, or to lead a climb. Are they rock climbers? Not really, IMHO.
Bottom line, for me, though, is to just get out there, be safe, and have fun. Hard climbs are stressful. I've had, and still do have, the best times climbing more moderate routes, say 5.6- 5.7 or so.