I think the comment about the line of sight being higher isn't about the stock, its about the height of the sights over the line of the barrel. The sights being higher than the barrel mean that to get over a berm or cover, you are more exposed than if the sights were closer to the barrel, and you werent as tall above the cover to use the sights. I dont know how much difference that makes to others, it doesn't much to me, I hardly ever get into firefights, and dont anticipate basing my gun choices on that issue. I like the general ergonomics of the AR types. I was surprised the first time I tried a shot at a running rabbit and scored the first round. I repeated that many times when I fooled around with them. It's all academic to me, I don't own an AR.
The AR isn't alone in the height of the sights over the barrel, the AK preceded it by several years.
I agree with Kraigs assesment of ergonomics. Many guns seem to fit me well without much drama, and a few don't. The ones that fit well are easier to use well, and are generally quicker to use.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-