1. What factors do you consider important in causing mixed ability? Put these in order of importance for you:
previous exposure 1
attitude 5
learning style 3
personality 6
aptitude 4
motivation 2
From my own point of view the previous exposure is vital, if you haven't seen some topics before you are not going to succeed so much. Altghough motivation and your learning style can help to improve and get the level the rest have.
3. "Every group is a mixed-ability group." Do you agree with this? What are the implications of this statement for teachers?
You are completely right. I have been teaching for more than 12 years and every single class I have had was a mixed-abiblity group. Noticing or not is also something the teacher can do or not. You can decide to prepare a single class for all the students and it is their duty to adapt, or the teacher can help adapting and changing a bit.