tbc changed itemization formulas so that stamina "cost" less on an item, therefore TBC items had HUGE stamina boosts on every single piece of gear, making it worth grabbing green/blue stuff over much of pre-TBC -- although T3 was still decent going into Karazhan. Also TBC introduced gem sockets.

in contrast formulas haven't changed and there are no new things, so its a minor progression. Even still, by the time you 74 you should start seeing upgrades, after all, the first zones are meant to be doable by people in green 68 gear so it can't be massive upgrades.