Setting difficulty level of classes

I've copped a bit of criticism about the level of difficulty of my classes. Some students have found it too hard, others too easy. This to my mind is an indicator that I'm probably doing ok.

After all it's better to push kids too far and get the difficulty level wrong occassionally than not push students and get the difficulty level wrong all of the time.

I try and manage the difficulty level through a number of factors:
1) Are the majority of kids engagaed in learning?
2) Are mean scores in tests increasing?
3) Can those not engaged be moved next to engaged students?
4) Can I make the course more difficult?
5) Is the top student being extended?
6) Are students endeavouring to pass the top student?
7) How long have we been on the current topic?
8) Where are similar classes at?
9) Have I covered the intended content?

And so on..