Not Enough Thinking
Far worse, I think, than too much thinking, is not enough thinking.
People who play automatically rarely play an interesting game, and don't seem to improve very fast. When someone "thinks" for a long time and makes a bad move (Too much thinking), I'd say they were not really thinking, that they were just being indecisive. Indecision is certainly a bad habit, especially in its worst form of wanting to take back moves, but playing slowly, calmly, and decisively, with plenty of time for both parties to make their moves without feeling rushed, seems to make the game far better. Fast players often seem to play boring hands. I'd rather play a two hour game with a thoughtful and creative opponent than four half-hour games that are simple and shallow.
Skelley The problem with amateur players is that thinking doesn't really help them a lot. If you have your games analysed by stronger players you'll notice that a 'profound' 2 or even 5 hour game does not really produce better play than a 1/2 hour 'shallow' game. Sometimes you need to 'calculate' a particular sequence ofcourse, but thinking in general just confuses you. I often teach 5 - 10 kyu players and they always tell me stuff like: "I was thinking I should play there or there because white could play such and so, so then I would have to play there anyway, but on the other hand if I play q18 now then I can't play it later, but white will play r17 and I..." and so on and so on. And in the end the move they played turns out to be the product of their thoughts: nonsense. Then I ask them to look at the board, and point out their weakest group and their opponents weakest group. This usually only takes a few seconds for them to answer. Now, where should you play? Don't think, look!
Incarlight Isn't looking without thinking as bad as thinking without looking?
Larsen The issue is not that thinking for a long time produces much better game (given that we talk about 5-10 kyu players), but that a game becomes "different". One reaches positions that are not the same as those played habitually and has to cope with them. As for me, I noticed that playing a long game I make "totally different" errors. Nowadays my first instinct is to play for some safe territory and then invade. Only having more time I begin to think about influence, keshi, light shapes and so on.
lavalyn I've continued my habits of "not enough thinking" (really, playing too fast for my own good) but I notice I'm making different kinds of mistakes. Flipping from one extreme (heavy) to the other extreme (causing opponent to become overly thick) solely by matter of observation post-game "this didn't work" "that didn't work" "I got chased around like crazy after this connect" "I didn't get enough compensation"...
Playing "in your comfort zone" is probably a worse habit than not enough thinking. It might win, but it won't help you improve.