I disagree. Anything up to 2 lb per week is a reasonably healthy rate of weight loss, especially if you're quite overweight to start with. Losing 40 lb in six months is both perfectly healthy and a damn good achievement.
When I cut soda from my diet I lost 10lbs in a month. In a month! And then it tailed off and I was depressed because I couldn't sustain it, I had unreasonable expectations. And if you're losing 2lbs a week for a very long period of time, it can be an indicator of underlying medical issues.
In short, don't get depressed about the scale. Look in the mirror and keep making body composition changes.
I would say 10 lbs a month is about average for someone who is motivated and losing weight.
Much like matwood, I was ~205 at 6' with an athletic build. I wasn't overly fat, but hefty (ok, kinda fat). Running and working out six mornings/week and basketball in the evenings allowed me to drop 2 lbs/week, to a much leaner 165. Coincidentally, I had a doctor visit or two during that time and the doctors never expressed any concern over it.
Fortunately, and I believe this generally for healthy adults, you will reach a plateau. I did so at 165 and never fell below that, even when I increased my running, lifting and other recreational activity.