So Much "Harshness"

Harshness can also be caused when the drivers (speakers) are stressed. Insufficient power can create this problem, whether using tube or solid-state amplifiers. One remedy to try is heavy-gauge speaker cable, i.e., 4 gauge, 1600 strand copper (often marketed in raw form as power-cable).
This will generally ensure a delivery of power, and a power reserve, for demanding musical passages when the drivers are under high-stress. Installing this type of cable can eliminate the harshness found at high frequencies under most circumstances.

02-22-09: Commcat
Harshness can also be caused when the drivers (speakers) are stressed. Insufficient power can create this problem, whether using tube or solid-state amplifiers. One remedy to try is heavy-gauge speaker cable, i.e., 4 gauge, 1600 strand copper (often marketed in raw form as power-cable).

Seriously dude?
I ran 8 gauge cable for an oven when remodeling our kitchen a few years ago. That stuff is extremely stiff and difficult to work with. How in the world are you going to connect 4 gauge to your amp and speakers. If you have done it please post your system with pics for the world to see.
Now you got me wondering what gauge cable is feeding my house from the transformer.
As for harshness, I agree with Mrtennis (can't believe I said that). In a general sense I see the culprit as metal dome tweeters squeezing that last little bit of resolution.
I don't listen to super systems so my opinion only relates to real world equipment.

Tim,
The 1600 Strand, 4 Gauge, is very flexible. Easy to solder either ring or spade terminals or any other terminal to the wire. Sold by David Levy & Co. in Cerritos, California or PartsExpress.com and others for under $2 per foot. Check under the Power Cable category. Basically the same cable as marketed by many Mega-Buck speaker cable companies for $1500++.

"Why are complaints of harshness so common".....

Because so many of the folks that make these complaints are flying blind when it comes to component selection, system synergy, and system set up.

They base their selection on professional reviews, manufacturers puffery, recommendations of dealers and salesmen, and fantasies (thiers and others) of what is possible in thier budget and environment. Often they have no knowledge of creating system synergy with a specific end/goal in mind, little knowledge of what is possible, or not, and know little about proper set up. If you think otherwise, you're not reading a lot of questions posted here. And answers as well!

We live in a society where most so many casual hobbists just want an 'answer to fit' without doing any research and accepts the answer which sounds best to his mind, not his ear. He makes himself a mark.

Synergy, is often created when you join a less than perfect component with another with complementary features. Its the old a 'brightish CDP matched to a rounded/warm pre-amp/amp'. Not a brightish CDP matched to a high resolution SS pre-amp/amp and very 'neutral' high resolution speakers as well. But then you must allow for the fact that I don't believe in 'perfect sound reproduction' nor fairies, so its easy for me to say. It would not be so easy if I had to make a living in this hobby.

And FWIW, I agree with Tvad re digital v analog. I think Mr T wasn't listening much to music in the pre-digital days, and I agree with Mikelevange - harshness, whether from distortions or non linearities, has many many potential sources, which we all hear differently and or value differently.

FWIW - I just felt the urge to rant a bit..........:-)

Commcat,
Are you sure you don't mean 14 gauge. I looked at Parts Express and the largest cable I could find was 10 gauge cable. Here's a wire gauge chart from Wikipedia. Not looking for an argument but 4 gauge is huge, hard to believe it's flexible.