I don't put a lot of emphasis on achievements, but they're handy for a few things - I'm pacing my journey through Northrend by not leaving a zone until i have the quest and exploration achievements. It's worked pretty well - I get all the quests, and a lot of them are worth getting to, my latest favorite is the laying of mines in Storm Heights, and riding the rocket - fun stuff!

I may go back and get all of the instances in Outlands, but I can't see taking 80's to Wailing Caverns.

However, I'm going to be starting some new toons soon, so I'll make sure to run them all through all of the instances on the way up!