I have beaten the entire game, done a ton of extra quests, but I am still only a level 47. I think this is because the quests in dragonspyre don't give enough XP. Most quests only give around 500-600 XP! I think KI should fix dragonspyre and make the quests give way more XP. Also, they should give the right amount of XP to players who have already beaten dragonspyre.
you may feel like you have done a ton of extra quests, but you missed a lot more. My Fire wizard skipped all Marleybone quests (except main story ones), has not done any quests in Grizzleheim, and has Stone roses / Malistaire quests left in Dragonspire, and he is level 49 (a couple notches into it). There is a side quest in most every area of the worlds.
If you do every quest there is, you'll be Level 50 just a bit before you reach the final area (where Malistaire is). I know that because I did everything outside of Grizzleheim except Kensington Park, and I got Level 50 a couple of storyline quests before The Final Countdown.
You must have skipped things. The only way to find out is to go through all of the various areas, looking for characters with exclamation or question marks over their heads. Some likely possibilities are: the Prospector Zeke quests in each world, the book quests in many worlds, Colossus Blvd, Sunken City, and The Beguiler. Also, Mooshu has many areas where the NPC who needs something is clear down at the end of the road. You might want to work from Dragonspyre backwards, since Dragonspyre quests give the most experience.
Going back may seem like a lot of trouble, but you'll spend a lot less time backtracking and getting experience from quests than you will trying to farm or level up on enemies.