Now there are certainly advantages and disadvantages to both. Uninstanced dungeons create a very epic dungeon experience, as you can come across other small groups of players fighting against the hordes of monsters while you explore a huge underground space. However, uninstanced dungeons can sometimes mean overcrowding, lines to kill bosses, and generally non-immersive behavior.
Instanced dungeons, on the other hand, let players enjoy the content at their own pace with no kill-stealing and no lines. But instanced dungeons don't seem to have that same flair for adventure and unexpectedness as a non-scripted dungeon does. In short, it's like riding a pre-planned roller coaster rather than having a true dungeon adventure.
So what do you prefer? Let us know readers, let us know!