I disagree, Overplanning is a bad idea, but planning in general depends on the style of writing you are doing. Writing a Mystery or some kind of psychological thriller or horror; if you don't plan it won't make any sense, at all.
I personally think Basic Planning; essentially to have some basic outline is a requirement but under planning can be a problem especially in longer works because you might not even see how you can draw a clear line to your climax and then to your conclusion that you want to make without making huge plot holes in the story.
Character Planning on the other hand is critical, you have to know your characters better then you need to, to make them realistic of any sort. If you don't think a character through they won't seem like real people. (i.e.: a robber who just robs for no exact reason or depth in his personality.) If that is the case, the reader might just lose any appeal, feelings or interest in that said character.