Can someone explain why Luke is so ridiculously overpowered after the ROTJ and how did he manage to get there?
Too many EU authors, Kevin J. Anderson especially, took Vader's statement that "The ability to destroy a planet is insignificant next to the power of the Force" a little too literally.
"Use the Force, Luke!" became the get-out-of-jail-free card for every bad and/or lazy author who wrote Star Wars between Tim Zhan's two Bantam entries (Heir to the Empire/Dark Force Rising/The Last Command trilogy, and Hand of Thrawn duology.) One of my favorite parts in Hand of Thrawn actually occurs near the beginning of the first book, Spectre of the Past, when Luke and Han are trying to figure out how to deal with a pirate ship, and Luke is trying to (in Superman fashion) decide which of his crazy super-powers he'll use to defeat them. . . tear the ship apart with the Force, take control of the minds of the crew and make them sit still while they're arrested, whatever. That's when he sees a vision of Emperor Palpatine and Exar Kun laughing at him, and he realizes that maybe getting flashy with his Force power isn't the best example to set for proper Jedi behavior. A good chunk of those books is devoted to Luke gaining new insight into the Force, specifically the idea that it shouldn't be a hammer to pound every problem into submission.