I warn people during the teacher training that leading to beginners is harder than they think. Leading a beginning yoga class takes some skill and understanding that leading to people already used to a yoga practice takes to a different degree. At first it seems counter-intuitive - beginners can't "do" as much because they have a lot to learn, so it'll be easier. It's not. Trust me.
Let's look at a beginners class...
People coming to a beginning yoga class often have not done yoga before (there are lots of exceptions as I have had the same people coming to my beginning yoga class for years) and so they are coming for a reason. Do they have a physical issue they want to correct? Did someone recommend yoga? Chances are they're coming because there's something wrong and they want to fix it. They've heard about yoga (who hasn't?) and they want to give it a try.
So the teacher is going to need to be familiar with typical issues people come in with - sore backs, being overweight, tight hips, lack of sleep, arthritis, injuries, knee pain - etc. And the teacher has got to be willing to NOT deal with those things. Be aware of them but not treat them. Everybody wants a simple exercise that they can do that will help their back, neck, belly fat, whatever. It does NOT exist and the teacher needs to resist the urge to fix people. We need to be prepared however, to accommodate their situation with alternate moves so they can get themselves into the poses without further injuring themselves.
Then we have to deal with the fact that people in beginners classes have not already been doing yoga in general (see above for exceptions). People who are not already in a habit are going to possibly be harder to "convert" to a new habit than people who are already in a stream of doing that thing. Same is true for getting books read or other habits in place - how many books do you have on your shelves that you started reading but didn't get through? It really takes something powerful to get people to keep coming back to yoga.
If you've got people in your class with pain and they start a yoga class, chances are they are going to start feeling more of their pain at the beginning and this can be an unpleasant process. As people get more aware of their bodies, they get aware of the discomfort they're in and to get over that and be willing to be faced with it weekly or however often the class is, and it's tricky.
Sometimes people are super-motivated to come to class because they've had a huge change in their lives like being diagnosed with cancer or is losing someone close to them and they will do anything to relax and try to line things up because there's a lot at stake. So they're serious, they don't want to mess around and they don't feel they have time to lose. Those guys need to be handled so gently and offered such a big space to relax and be - holding space for people while they're grieving is a tall order. Once you're trained in doing it it's an immense privilege, but along the way, the yoga teacher can come face-to-face with her own inadequacies and unresolved fears and grief, too.
Beginning yoga students tend to do what you tell them to do as best they can and will try to copy the teacher. If you're a new yoga teacher there's a good chance you like yoga, you're "good at" yoga and you've been doing yoga lots longer than your students. If you go up to the front to demonstrate your full version of the pose, your students are going to knock themselves out trying to follow you. It takes something for the teacher to be aware of her full expression of the pose and resist the urge to go there while guiding her students. It's one thing to inspire and motivate them, but the line is so fine as not only will your students possibly hurt themselves copying your form, they'll think they're supposed to look like you, the teacher, and they're not. They're supposed to look just as they are.
I had an experience this summer while I was at Omega where a famous workshop presenter buckled under the pressure of leading to a diverse group. Her workshop basically fell apart and I was called in to help pick up the slack in the yoga teaching area. When I asked her what happened, she said that the group had so many different expectations and were at all different levels and she didn't know what to do with them. "That's my speciality," I told her as I took her group off of her hands and led the last couple of days' yoga classes. At that moment I realized that even though I'm not the fittest, stretchiest, hottest yoga teacher around, the experience I have from being with beginners wherever they are at has translated into a skill that is useful to me everyday when I teach.