As a side point, it would be good to have a link to attendance in some way, even if only a change to the current system, to discourage people who don't bother to turn up (obviously not if there's some reason for you not being there). Particularly in first year, or in modules which people didn't like or found hard, the attendance tends to be really poor, in some of my lectures the vast majority of students would regularly not attend. Crazily people spend 9k/yr, rent a flat on campus, and then still can't be bothered to walk from the flat we can all see from the lecture room and come to class. They are wasting funding and places that other students would be very grateful for.
Not sure how this would work with a gradate tax. One big motivator (at least for me!) is that you're paying to go to uni, so you want to get the most out of it. Ok, so you'd still be paying back money forever, but would you see it in the same way as directly paying to go to uni? Would people just kind of recognise and accept that they're paying in some indirect way, and not have the same motivation to work hard? How would it work if you quit after 1 year/failed/dropped out? Fine details of the scheme (or others) may well sort this, but need good thought, and would make for an interesting discussion. Not that the current scheme seems to deal with this issue. Free tuition would certainly have this issue, it would be interesting to think about how poor attendance/lack of effort could be discouraged in this case too.