I realize that all this is in the eye of the beholder, but to me there is a big difference between high extensions (Suzanne Farrell, e.g. or, for her time, Sizova) and facile, loose-jointed opening of the legs in every direction (e.g. Guillem). Guillem can flaunt some very impressive physical feats, but does she have true ballerina line - that overall, shifting but centered harmony of movement as she goes from feat to feat? Not to my eye, especially compared to, say, Zakharova. Zakharova's extensions are often super-high, but they remain in proportion to the scale of the rest of her dancing. That's what is critical - not the height of the leg or foot, but how it fits into the rest of the dancer's line and performing.