Leave positional alone please. :-P

A good tank will not only keep aggro, but also keep the mob(s) positioned accordingly. Depending on the fight, repositioning may well be needed, but the party should be aware of this, assuming it's not their first time in and they didn't do research first. That said... you got your bad tanks as well as dps that clearly don't care if they get aggro or not and fail to use abilities that lower their enmity generation. In cases like that, you just have to adapt and do what you can.