So, here was my experience in 2nd Edition. That's a really good question, too.
I had run 1st Edition (in the Forgotten Realms). I ran my players through a "Avatar" series of adventures to mod their characters and transition from 1st to 2nd edition. I think my players would agree that this was a favorable and memorable experience. Overall, we ran several really cool campaigns. I ran up as high as the "20's" in 2nd edition. I explored about every book that came out for the game. I enjoyed this game, and there was plenty to play with as an amateur designer/tinkerer. One of my favorite campaigns was near the end of 2nd edition with teh Faiths and Avatars books from FR, along with the great "higher level" modules that were being written including Axe of the Dwarvish Lords, A Paladin In Hell, Rod of Seven Parts, and Dragon Mountain. Encyclopedia Magica and the spell compendiums are among my favorite books ever published.
My challenges started with the kits. I LOVED the kits and the complete books first came out. It seemed like a needed and clean way to balance players of non-multi class characters with the multi-class characters. Plus, it was a cool, and flavorful way to flesh out a character in an RP'ing way. The complete dwarves handbook came out, and I actually wrote my first letter to Dragon Magazine (forums) and got published! I was SO upset that they unbalanced, and were about to continue to unbalance the game from there. Which they did... The Skills and Powers book was simply too broken to use straight up. The charcater class balance was really tough to judge through the levels (from 1 - 20) so I was constantly pulling powers and abilities in from other games including Rolemaster, Dangerous Journeys, and Fantasy Hero. I had fun doing this, but it was extremely painful to have to do all the work, and not really be able to "share" these experiences with other gamers online or at conventions.