The hardest part I found, was the classic "wall of silence". You spend 10 minutes explaining something new, and then ask if they are all happy and understand it, and are met with silence. No responses of "yes", no responses of "no".
I hate it, as there is no way of telling whether you need to go through it again or try a different approach. You can't assume they don't get it and do it again, because if they do get it, you'll just waste time and bore them to death. At the same time if you assume they do get it, inevitably 10 minutes later you get asked questions on the subject halfway through something else, leading you to need to then repeat what you were just saying as they were thinking about the other topic and not what you just said!
The feedback from the course was all really positive, the only "negative" was one person ticked the 3/5 box for material covered in enough detail. At the end of the day I was asking them all whether they understood everything and/or wanted me to go through things in more detail, so I'm not really concerned