Character Derailment/Headscratchers

It seems that there is a fine line between character development and and character derailment. Generally defined by whether or not we "like" the change. Examples:
 * Ben 10: Alien Force
 * People thought the "new ben" was too mature compared to his 10 year old predecessor. I think it was reasonable that, after four years of living a "normal life" you'd be thrusted into a situation where one of your loved ones are missing/endanger so you have to assume your super form, one might become more serious in light of that fact.
 * Sonic the Hedgehog
 * People praising Sonic Colors for rerailing his classic-esque characterization (wise cracking jokes etc.). Yet isn't modern Sonic supposed to be older? So his lack of wise cracking in the post Adventure era could be seen as character development.
 * In the first example, it's no so much that he matured as it was that he now had no personality at all. With the latter, Sonic's attitude was part of his appeal; just because it changing is plausible doesn't mean it's a good idea.
 * OP here, but because of it's "plausibility" shouldn't it be considered development as opposed to derailment? Regardless if it's a good idea or not?