I think the point is not that it has to be a huge time sink but the really awesome gear should not be something you can get in an hour of play. As long as casual players can log in and make meaningful progress in a short space of time there is nothing wrong with that. So if a hard core player can complete something in a week, then a casual player may take a month but they can make some progress everytime they log on.

I liked how Nyzul Isle worked in FFXI it was challenging and if you were short on time you could get together and just do 5 floors and start from there next time you logged in. The trick though is to make it so players will still want to run it even after they have what they want so you will always have people to go with.