Sorry, I disagree. Marleybone is the worst and longest and the MOST BORING. WAY too many side quests. I would like to see all of the side quests from Abigail and the one for the Date Letters DROPPED.
The side quests given in the different areas are ok as they are in keeping with the rest of the worlds of the spiral.
Grizzlehiem and Wintertusk are side worlds/quests. You don't even have to do it in the first place. But, the experience given in this world is amazing. I personally think it is worth it to go through.
Because I finished Grizzlehiem and Wintertusk before doing main worlds, I was level 62 when I did Mooshu. Went through Mooshu very quick thanks to that.
The side worlds are great to get experience so that the main worlds that are apart of the plot are easier.
Another example, I just got done with Dragonspyre today at level 66. Just unlocked Celestia. I am flying through these worlds quickly.
:P 
