I agree Raids have nothing to do with D&D, and going the other way they are pitifully small for MMO Raids.
The problem with quest chains is that they are a large time commitment. Unless you have a static group, starting a 6 parter at part 3 is very very very tough. It's tough enough to fill 1 empty spot if somone drops. Many people just don't want to joint TR half way through because if they run it, they want the end reward.
Yup, its better D&D but unfortunately it doesn't seem to work well here...