Judgment is a really difficult skill to teach. Many boaters take to this sport like bees to honey. Often, they want to progress up the river class scale as fast as possible. Quite often they assume that just because they have a dynamite roll, they can run just about anything. These types of paddlers are an accident waiting to happen (and happen it will). Some recognizable traits are:
Although this can happen to younger paddlers, middle aged and older paddlers fall into this trap as well. People that have been paddling a very long time and are getting older are subject to another trap. As we age, many of us fail to exercise enough to stay in shape. Our reaction time gets slower as well. Many kid themselves that they can still paddle as good as when they were younger. At first, you end up swimming more often than you used to. As we get older, our balance starts to degrade. It is even more important for older paddlers to increase the time they spend on exercise than when they were younger. Unfortunately, it is often hard to find the time to do so.
There is no doubt that boating gear has come a long ways from the "Good Old Days". With proper training, the learning curve for boating has shrunk a great deal. An unfortunate side effect of this is too many adrenaline junkies really pushing the limits of their skills and equipment. More advanced boaters have fatal accidents these days than when I started boating 30 years ago. Great boaters pushed the limits of their days but did so more methodically - baby steps. For a really sobering education, check out the AW accident database: AW Accident Database.
So how can you identify a paddler with good judgment? This is a really good question. Paddling is like going to an auction, you really need to determine your final bid and walk away when the price is too expensive. In a paddling context, your final bid should include a certain number of swims - say 3 strikes and you are out - time to take off the river. When you need to watch everyone run a drop before you are ready to take the plunge, you probably should walk that drop. If you are having an off-day and can't seem to hit lines that are normally easy for you, consider walking the more difficult and dangerous drops - even if everyone else says you can do it or you have done it many times before. Peer pressure can be really dangerous in this sport, you sometimes need to develop a thick skin. Whatever you do, try to avoid applying peer pressure on others. Good judgment is the hallmark of an expert seasoned boater.
The American Canoe Association has put together several excellent articles in regards to judgment.