I've read two great posts about this recently, one from Jason Fried and a response from Dustin Curtis.
I'm writing this, in part, because I need to be better about this. I'm confident and think quickly, and I can often find a flaw with a plan pretty quickly.
It's really easy to shoot down someone's idea or plan. If you think quickly on your feet, you can often find a reason to trash an idea before you've even had time to think about it. I do this a lot and need to get better about it.
There are three problems with this sort of attack:
1. Maybe it isn't a bad idea. Maybe it's a good idea with a flaw. If you actually digest the idea, you might be able to improve it rather than discarding it. And even if it turns out to be fatally flawed, there might be something good to it that you could use later.
2. Some people don't like having their ideas slapped down like that. Sure, there are more combative people who don't have a problem with it or even enjoy that sort of exchange. People are different, and this is a case where neither answer is right or wrong. It's just different. And the people who don't like this are not particularly likely to tell you about it. In a lot of cases, they'll just stop sharing ideas with you.
3. To most of the people who don't like to be shut down like that and to some people who don't mind so much, this makes you look like a pushy jerk.