From the starting zones all the way to the last quests you do before hitting 80, most are tuned to be easily soloed, but not crazy hard. Players can blow through quests at a fast pace. And there are soooo many of them! Other then just the difficulty tuning, I have to say I am very impressed with how well the quests are laid out. There are tons of quests where you start out heading for Objective A, along the way pick up a quest to go to B, where you additionally pick up something for Area C. You get sort of lost and absorbed in these great little side trips, with a bunch of extra quests before finally getting back to A.
There are only a few of those chains that send you back in to the same area to kill the guys you just killed, but there are a few. I hate those. Its like "Hey, I just killed a dozen of those things to get to the stuff for the last quest, and now you want me to kill more of them?" Fortunately, there aren't that many.
And the quests are generally a lot more interesting than the standard gather-pig-spleens quests of old. The quests where you ride a dragon, or a boss, or a vehicle are just a blast!
In general, WotLK quests are very nicely tuned, well laid out and fun.
The leveling dungeons are tuned to be easy, possibly a bit too easy. That may not be such a bad thing; lots of people got to see lots of dungeons while leveling. And wow, most of the dungeons are just amazing! The visuals are awesome, and many of the boss fights are more creative by far than the older dungeons.
Once you get to the heroics, difficutly ramps up quite a bit. I was continually amazed at how tightly they were tuned. I was mostly healing the first heroics we were doing. The margin for error was slim indeed. I would run out of mana often right as the boss died, and had very little downtime or margin for screwing up. This was with fresh 80s like myself, all of who I consider to be above average skilled players. Once some heroic gear started piling up on us, the heroics became much easier quickly. The difficulty of heroics is tuned very tightly for fresh 80s, extremely good tuning job by Blizzard.
This is another area where WotLK is tuned very well in my opinion. My Druid was wearing pretty much full Tier 6 coming in to the expansion, with several best-in-slot pieces, both Resto and Boomkin. Don't think I replaced any gear before hitting 80. At 80, heroic gear started replacing my old stuff really fast, and rep gear as well.
This indicates to me that Blizzard designed a 'parity point' of sorts, a point that all players would reach where their gear would be close to the same level, whether they entered the expansion geared to the teeth or wearing greens. That point seems to be very shortly after you reach 80. I find this an amazing feat of tuning.
My Hunter is 73 with now, and was geared with Kara and Badge gear. I have replaced 1 piece of gear on him so far I think. The quest reward gear is getting very close to being upgrades. In many cases, the only reason they aren't upgrades is the enchants on the gear he has right now. I am not going to put expensive enchants on leveling gear of course.
Other than the raid content being tuned to be too easy, I think that all other aspects of WotLK are tuned amazingly well. I am hoping that the next raid instance released will be a good chunk harder.