now that 6th ed has been out for a week, and we've all played a few games, what are your ideas on the tier system (without thinking about allies).

I'd say along these lines

tier 1
grey knights, imperial guard, necrons

tier 2
blood angels, space wolves, orks, dark eldar

tier 3
space marines, dark angels, eldar, tau, chaos marines

tier 4
daemons, black templars, adeptus sororitas

tier 5
poor, poor, nids