1.Motivation
Previous learning experience
Learning style
Attitude
Personality
Aptitude
2.I think is hugely important to figure out what makes a student "tick". Once the teacher has pinpointed the student's motivation, she or he can adapt to the group's needs. Aptitude can sometimes be a drawback, as students who learn easily tend to get lazy or bored; additional tasks may be created for them, to take them "by surprise" and keep them focused. Other suggestions may be different homeworks, switching pairs or personalized tasks.
3. All students are individuals with different backgrounds, personalities and aims. teachers should never forget that a new approach may be suitable with each new group of students, regardless the fact that the course book might be the same.
4. Students of different levels can either compete with each other and that I found beneficial for the group itself, or they can complete each other and working together improves their skills. In a nutshell,if the students' level don't differ very much, mixed-ability classes present obvious advantages for the students.