The objection that, in the midst of a long document, telling what level of indentation one is at would be difficult is easily addressed by having the editor sho

The objection that, in the midst of a long document, telling what level of indentation one is at would be difficult is easily addressed by having the editor show space marks or using 'fold lines' in print -- neither of which is difficult. In practice, though, I never have any problem with indentation based syntax, because change in indentation is easy enough to detect, and that's good enough for anything I've ever had to read.