Kotkaniemi is an example of a player ruining his own development.
He was given every opportunity and failed to make the most of it. Usually I blame the team for not developing players right but I really don't know what more the Habs/Canes could've done with him. He's been constantly put in positions to do well, Habs pretty much stapled him to a third line role between Lehkonen and Armia, guys that were from the same country (Armia was from the city next to him). In his first offseason, Habs sat him down and set up skating coaching for him that he declined and instead made his own workout plan to bulk up.
The reason he wanted off the Habs was because he got scratched. He got scratched in game 5 of the Stanley Cup Finals after spending nearly the entire playoffs being put on the PP, in sheltered offensive minutes, and failing to make much offense. He was scratched after going pointless in 7 games and being a liability, he was only 20.
I'm not surprised he never improved since his rookie season, he clearly doesn't have a perception of his own flaws and a willingness to work on it.
If you ever want to read on why he never became an impact player, Jack Han had an amazing article a few years back predicting his stagnation. Han absolutely nailed it that he never had it in his game to develop into much more than we saw, he goes into player body coordination and body matching reflexes, it's really good.