I've never played another MMO but I'm (possibly mistaken) under the impression that much of the leveling in those MMOs takes place in zones like the DDO explorer areas. Doesn't really happen here.
Here's the thing . .. explorer areas can be awesome . . . once. One of my fondest DDO memeorys was day 2 or 3 of Motu getting lost int he Underdark with a party trying to find the Drow City. It took us like 3 hours and it was a blast.
But . . . Explorer zones offer little to no replay value. The whole schtick of DDO is REPLAY value. This is a grind game, a crazy grind game where XP is the most valued commodity by far.
Shadowfail came with a enormous explorer area . . . but no raid. All the work that went into the Stormhorns was wasted time for something people run around in once. Why put all that work into something with no replay value?
So how do you give Explorer zones more replay value?