Add genetics to that list. And really, they're the basis for many of the factors you stated. Our genetics will affect a wide variety of variables (our fiber composition, sensitivity to certain anabolic hormones, production of hormones, ability for hypertrophy, bone structure, training capacity, etc.). In the grand scheme of things, people respond the same way to training certain ways, but training used for advanced athletes would likely be too great a stimulus for less experienced trainees, while training for the Average Joe is not adequate for an elite athlete. And even at the elite level, people display vastly different responses to training due to genetic differences. Thus, while the overarching training plan is similar across individuals within a certain category, implementation takes on an individualized aspect that one must account/adjust for when training. Hence the art of training.
I will say, however, that continual evaluation of a training protocol is how it is validated/rejected (returning to the science of training). If a superior method is created and validated, less superior methods should be discarded in favor of the superior method. I think that's where we go wrong in this country. Something sounds good and trendy (my boss calls this the Kia effect), but it's un-validated and no one really bothers to do the research on it to determine if it is superior. So we hop from training program to training program, hoping something will work. All the while, validated methods are out there, but they don't sound as trendy or exciting as...well, I won't mention any names.
As an aside, the Kia effect: You're at a car dealership where a Kia and a Mercedes (both brand new; it's a metaphor, stay with me) are the same price. The dealer hands you both pamphlets, and you read each one. "Oh," you say, "this Mercedes has all these features. Looks pretty nice." But then you look at the Kia pamphlet with the trendy hamsters on the front. There are all kinds of colors and it seems flashy and attention grabbing. "Man," you say, "this looks really cool. I want to test drive this." All the while, the salesman is standing dumbfounded with the Mercedes keys in his hand, originally thinking there was no way on God's green earth you would want the Kia. Yet you test drive it and buy it, thinking it's great. Only, a few weeks down the road, it gets boring, so you trade it in for another car...and another...and another...and...
Sound familiar?