This system is inherently catered to the individual while being one-size-fits-all, because in the end, we are all the same.

I practice ashtanga yoga in Mysore, at the main shala.  Sharath is my teacher.  

I have the honor of assisting  him and working with 80 or so of the lovely aspiring yogis who come to the shala each morning. I have the privilege of attending conference and laughing at Sharath's little jokes about fear and pain and opening.

Many of us who have dedicated our lives, or at the very least our mornings, to this practice have come to see the brilliance that is the series of postures.  Not only the poses, but the vinyasas and the nuances of breath and drishti. For the poses I have completed, I frequently notice the importance of the order and the importance of establishing each in the physical body before moving on. 

and I've seen Sharath throw these rules out the window.

I've seen him allow a man with pins in has back to practice full primary without binding in marichi d or supta kurmasana.  I've seen him allow a woman with only fear in her way move onto to intermediate (thus giving her the emotional encouragement she needed to stand up from backbends). I've seen him stop a flexible student for no reason other than a lack of stability. as well as for a blatant display of disrespect.

The series of ashtanga yoga is a system that serves as a base and a structure for an experience of transformation.  an experience of yoga. 

Sharath tells us weekly that asana is not what is important.  It doesn't matter what series you practice or how good you are at handstands. "That doesn't make you a big yogi," he is fond of reminding us. All he really wants is for us to love God, "any god you like." 

and he wants to be available for as many of us as he physically can; he gives more to us than seems possible or sane. To protect us he has created some general guidelines for progression in the series or asana (guidelines which he occasionally tosses, when they don't work for specific student) but guidelines that often make us stop and face things in our bodies or in ourselves that we might rather ignore. challenges or stiffness (in muscle and spirit) that we might choose to try to force open.

and Sharath doesn't care if we get more poses.  I have seen that he is happiest with his students who try and struggle and SMILE, and they aren't always the ones with the prettiest postures.

Sharath is real with us.  He acknowledges that pain happens. and in the same moment he tells us his own stories of injury and stepping back, of practicing primary for months to rehabilitate injury, of backing off of backbends or taking it easy when teaching.  and he tells us that pain is opening.  NOT that pain is the way to opening or a necessary part of the process, but that suffering will happen and that we will survive and learn from it.  that we shouldn't give up just because it gets hard or painful.

It is a subtle message delivered in abbreviated English.  
pain is opening. tapas is a vehicle for growth.

Change, opening, development and transformation all are accompanied by some level of discomfort,  whether it is in the fibers of the physical body or in our hearts and minds.  
Implying that the postures we do determine our experience of yoga is a gross misunderstanding of the practice.  Thinking that we have to practice to a certain point or continue to move forward, physically, without confronting our limitations is completely missing the point. As we change and grow, the practice changes and allows us new challenges.  If (when) we hit a wall, then that is where we can do our work.  It doesn't matter if it is half way though primary or at the end of fourth.

This is the tradition of ashtanga yoga.
It is not what order poses come in.
or how well you have to do them to move on.
or how many people are in the room when you practice.

It is the willingness to face yourself and trust your teacher. to face your ego. to accept the work you have to do.