The interwebz have been full of thoughts and answers to this question and it's something I've been mulling over for a while. I thought I'd add my two pennies for what it's worth. This is purely how *I* feel as a yoga teacher and bears no judgement whatsoever on teachers who think differently! :)
I think teachers, of any discipline, are enablers. We enable our students to reach their full potential. We encourage them to keep working, to keep improving. Sometimes our students surpass us. My mother was an English teacher for many many years and got many of her students into Oxbridge - a feat she never managed herself. Sometimes we just help our students move on to the next level.
As a yoga teacher therefore it is not imperitive that we can perform every asana we teach "perfectly" (whatever that means). In fact, in my experience, I have found that my own limitations give my students a deeper insight into their own practice. "Look at me," I say, "I can't do some of these asanas very well. Some of them I may never do in this lifetime, but I keep trying because yoga is a journey, a work in progress". I've always found this attitude makes my students realise what they are capable of, because they can see I am only human too.
Because to me what we as western yoga teachers are not is great leaders, gurus, spiritual teachers. I don't ever want to be seen that way. It makes me uncomfortable when students refer to me as their guru, or even their mentor. I am just an ordinary woman with an ordinary job who teaches some yoga because it is my greatest love. I am just helping you a little along your road in yoga. Most of the work comes from you, from deep inside. Don't become too attached to a teacher or a particular class in a particular place because this detracts from the root of yoga. That part of yoga that you carry around with you anywhere. That ability to practice, wherever you are, whenever you need to.
I am not denying there are some teachers who are able to lead the way, who are able to give deep insight into the spirit and philosophy of yoga. But if you are just an ordinary person who enjoys coming to a weekly yoga class to stretch and relax and meet people and ease your bad back then you really are the same as me. Eventually you will probably strive for more, you will look for further reading, you will self study, you will find yourself being kinder, calmer, less demanding. You may even train to teach yourself. But at the end of the day we are all the same, we are all on the yoga road. We are all in it together.