No one has brought up friction shifting on mountain bikes. I toy with the idea of putting a bar-top friction shifter on for the front der. It's tough to find good shifters NOS, and older ones can be problematic. Rivendell sells some that should to the job.
I like indexed on my MTB because I keep it in good working order. However, I think a friction shifter on the front would be superior to my Deore indexed shifters for going into the granny gear. A lot quieter, too, when cross-chaining a bit.