1. According to me, the factors that influence students in mixed-ability groups would be classified this way:
attitude
motivation
aptitude
previous exposure
learning style
personality
The most important thing for me is "wanting to learn", which I define as "attitude towards the subject". If they think they can do it and take English as an opportunity, not as a problem, they have all the chances to pass the subject!
2. I liked the idea of taking more into account the students' opinions: feedback about the lessons, the contents, the type of activities, the methodology...
3 and 4. Of course every group is a mixed-ability groups, you will always find students that are better at languages than others; students that learn better by listening and others that prefer reading; students that love speaking activities and students who fear them; students that are able to finish 10 activities while others are just in activity number 1. The problem is not mixed-ability students, the problem, or challenge, I would say, is finding an appropriate way of approaching and dealing with these differences. That is the difficulty we all teachers have, and it is not only planning the lessons and what you are going to teach, but also how are you going to do it in order that everybody can follow you.