for me it wasn't speed (that's purely down to leg power anyway)

My reasons to justify a better bike
disc brakes - the most important reason
better hubs, BB and headset so they should last much longer
bomb proof wheels so should support my clyde ass better and not need truing so often
serviceable campagnolo brifters rather than shamino "replace the whole damn thing" ones
it'll pay for itself in 5 years if I commute 5 days a week (that's not hard and fast though but it is a nice round figure)
custom frame fit and color