Their biggest mistake
4 years + of content.
Increase your levelling speed significantly.
Most dungeons you don't even go into when levelling no.
So you hit 80 and none of the 55 old instances/raids are of any use to you.
It would be nice, post patch 3.1, to focus on making the old content relevant to all those level 80s.
Give me Heroic Shadow Fang Keep. Update all the TBC heroics to be a challenge for level 80s.
Basically Blizzard are not keeping their old content relevant to their player base when they have mechanisms in place to make it happen.