I think it comes down to features and R&D. You'll see features like a decompression valve and available full wrap handles on the pro models (260 and 361 excluded). And you'll see consumer aimed features like Easy 2 Start and toolless chain tensioner on the homeonwer saws. Then it comes down to the amount of time and money Stihl puts into developing that saw. Why does the 361 weigh a pound less than the 290, but it still has more hp? Why does the 440 weigh only .4 lbs more than the 290/310/390, and blow them all out of the water as far as hp, torque, and longevity? The answer is R&D. The pro saws are also of a construction that is considered to be better than that of the homeowner saws-namely the vertically split case, compared to the horizontally split case.
Now I owned a 310 for a while, and it was a decent saw for what it was meant to be able to do, but I sold it and upgraded to a 361. The saw weighs less, has more power, and is just an all out better performer. And yeah, it cost me more $$ too, but definately worth it.
Jeff