Seth Godin used a visit to a car dealership to show that there is a very wide gulf indeed between a 'good' salesperson and a 'great' salesperson. I've been either working in a sales organization, or supporting our sales organization for three years now and I second Seth's observations -- it's too easy to settle for 'good enough.'
In a roundabout way, Seth also points to some of the contributing factors which allow 'good enough' to occur. In my case, our sales force has been selected more to retain existing revenues over growing new sales; it's part of the deal when you're the incumbent and you're protecting your base. But our sales compensation is so skewed toward guaranteed salary (and everyone making the same salary) that there's no real incentive to significantly grow business. Growing business means forging new relationships and that's riskier than working within your existing customer base. But if you're not growing, you're shrinking.
The corollary to Seth's argument is: If you don't pay the best, and let 'good enough' work for your competition, your 'best' will eventually be working for your competition.