With full credit to the old Bad Religion song, I've always found thinking negatively to be very positive--the positive power of positive thinking is also powerful, but it's an empty hope without building a foundation dug in the dirt of reality. Right now there's a surge of interest in this book called "The Secret," which advocates that all you have to do to receive something is ask for it and believe you will get it (something to do with a "law of attraction" which most physics textbooks don't apply to new jobs or plasma TVs). I see a lot of this type of thinking in the world right now, from the invasion of Iraq to American Idol contestants. I find this attitude of, "I want it, give it to me now world!" kinda funny in a little kid writing letters to Santa Claus way. Except that in life we had better not rely too heavily on Santa Claus, at least if we want to stay alive or in business. I sometimes give talks about how to deal with risk; if we simply hope for a good outcome when climbing or flying we're likely dead. No, we have to think "negatively" about what could go wrong and how to avoid that in order to mitigate the risks, or at least understand them.
I often think up crazy schemes and then actually do them, from climbing icebergs with Ben Firth to setting world records for paragliding so I'm all for big dreams, but I also know that those dreams won't become reality without understanding the risks before going all-out. It's like the old Boy Scout motto of, "Hope for the best, prepare for the worst." I believe in the power of a big idea, but I also believe that hard work and critical thinking will lead to success a lot more than just, "I hope it works out..."
There's a good article at Slate today on the whole subject, check it out.
PS--I just found that there's actually a book with the same title, haven't read it but sounds kinda interesting.