The biggest drivers for me are #1 (unreliability) and #4 (inconvenient hours), but also the teacher having a consistently bad connection. There's no point in taking language lessons if you can't hear the instructor speak half of the time, or if the connection keeps dropping that you can't sustain any kind of conversation.

Also, not quite #6, but I've also had teachers grow over-complacent with the classes when I've been working with them over a period of time (usually after six months), and start taking too many breaks, answering phone calls, "stealth texting" on their phones (not realizing that I can hear their phones vibrate each time and I can see their eyes look elsewhere), or chatting with others on Skype during the lesson (I can see/hear them typing often, but it's not being sent to our chat window). I stopped taking classes with them as a result.

@torusan

You're right about teachers who are texting or chatting with other people during the lessons but sometimes we have to be careful about judging too quickly because many teachers tend to take notes in a separate file to give a feedback to the student at the end of the session. 

Mark: All of these reasons are really comprehensive but number 6 was never my case (as a student). I never get bored of the person. I can get bored of the sessions (no results, no innovation, no motivation and so on). If the teacher is interesting as a person and keeps their lesson interesting as well, I don't mind seeing him/her 1000 times! :) 

But one thing that I would add is "No flexibility". I had a teacher that I consider her one of the best and she's very professional and she delivers a great job but I stopped having lessons with her because sometimes I was tired or not willing to have a "super lesson" and she wouldn't be flexible.to adapt to my mood and would kind of "force" the whole plan until the end to the letter.  

Me, as a teacher, when I see that the student is not in the mood for a lesson that will require a lot of energy, thinking or preparation from the student I simply adapt and go with the flow and do it according to their pace and motivation level of the day :) 

Not only about energy, motivation and willingness but also if I have a lesson planned, let's say about a X topic and then the student comes very excited telling me that he or she is getting married, for example, I won't act like "Oh cool you're getting married, now let's start our lesson about X topic..... " Sensibility and flexibility also play an important role during the session! 

I have 4 regular teachers and like to take occasional lessons with other teachers. I really like my regular teachers and can't imagine dropping them, unless I actually stop lessons all together.

However, when I was in the process of picking teachers, I did come across a couple that I would absolutely not take more lessons from. Some of those reasons were:

- No shirt. I don't do lessons with shirtless teachers.
- Smoking. One teacher leaned back against the wall and lit up a cigarette. 
- One teacher was barely awake. Yes, it was midnight where he lives, but then you shouldn't offer lessons!
- One teacher allowed me to introduce myself and then started telling me ALL about his life. I don't think I said as much as 10 words during the "lesson".
- One teacher kept giving me stuff to do on websites, so he could continue working on whatever he was working on. 

Those were a handful of bad experiences. The great majority of teachers I've tried have been absolutely amazing! The only reason I've not taken more than one (or maybe 2 or 3) lessons with them is because I already have a group of regular teachers.

I don't have enough experience with online teachers to comment, but here's what put me off some of my private tutors:

a) Using my native language more than the language I was learning.  I was paying the tutor to speak in her native language, not the one I hear every day

b) 1 (unreliable or late) - she kept cancelling or changing the lesson times, often without telling me, and then blaming me for "misunderstanding"

c) 8 (lesson material not suitable) - I was quite specific about what material I wanted to study, but they did what was most convenient for them (ie what other students were doing, so they didn't have to prepare anything new).  I cut my hours as a result of this, so that we could concentrate on what I wanted to study, not what they wanted.  I also didn't like it how my teachers did most of the talking during the lesson - if you don't get a chance to speak you might as well just watch a video.

d) 10 (teacher losing enthusiasm) - with one of my teachers it became apparent fairly quickly that she had no interest in my learning, and was just trying to maximise the money she made off me

e) Harsh criticism when I made mistakes - there's nothing that kills confidence faster.  And more often than not, the mistakes were a result of trying something that I hadn't been taught yet, so it was hardly my fault.

f) Not having a good enough knowledge of their native language.  What was worse, was she wouldn't admit her mistakes.

and not surprisingly, as a result of 1, 8, 10 and f:

3 (had an argument) - polite requests got me nowhere, so I started being firm, which led to arguments.