Thank you all for your comments, very insightful. Here are some more of my thoughts regarding "positive thinking", again according to my definition of this concept and how I've viewed it over the years (BTW I want to point out that I do not condone "negative thinking", my problem is with the "thinking" part)
One important problem I see is that it's based on a missunderstanding of how emotions work. Energies can never be suppressed, they can only be transformed. If you try to block/deny emotions through positive thinking (putting a veil on them, and thus hiding them from consciousness and pushing them in the darkness), then these energies get stuck in your body and you will have to release them sooner or later. During several years I repressed emotions of grief/sadness and it was very harmful, it's only recently that I was able to release them through recapitulation and self-hypnosis.
I want to thank Montalk about reminding me of this transmutation trick, I had read about it several time and tried it successfully but each time I have forgotten it for some reasons...? It requires a lot of concentration but it really works, I tried it again today and it's amazing.
To talk about another problem I see, I have to share a story. Last year I "fell in love" with a girl with whom I had a lot of things in common. Everything appeared to work wonderfully between us at first, however I was very afraid and used tons of positive thinking to move myself through this experience. I also did lots of heart chakras meditations, which I consider just as bad, since it forces to open this center in a non-natural way.
In the end it all turned out as a big failure. Not only she didn't love me, as I "positively thought", but she also heavily used/abused me. I think the reason why I didn't notice the abuse was because of the veil of "positive thinking" I had put. If I had just observed the situation and had faith that whatever had to happen would happen, it wouldn't have had to turn that bad. Also I wouldn't have put expectations so high and wouldn't have felt such a deception (I was far from balanced).
This brings me to another point. The reason why I met this person, as I later understood, was to learn a very important lesson about love. It was not meant to "work out", but my ego (which, of course, thinks it knows everything ) decided to force its way through it. So a real problem I see with positive thinking is that it buys into the fear/desires of the mind/ego instead of having faith in the higher/infinite self to guide us. (To take the charioter analogy, the driver stops listening to the master and thinks he knows where to go, which of course gets him lost.)
Furthermore, as it turned out in my story, the ego (conscious mind) does not have any control of the situation (even if it think it does), it's the higher self which defines the game. Thus most of the "positive thinking" is, as cameron pointed out, nothing but "wishful thinking". It makes us "feel good" (before making us feeling bad) but does not really affect important outcomes in our life. When our "positive thinking" does not work, we can always find new things to "think positively" about, but it's just going to keep on making us miserable and lost in the illusion.
There is, however, possibilities to really "create our reality" and this is by changing directly the unconscious, ie through magick. I have already mentionned it elsewhere and I think this is a very STS thing to do. For me, STS is about modelling our reality so that it fits the needs/desires of our ego, while STO is about modelling ourselves according to reality (and we do so by listening to our higher self which knows exactly where we best fit in the universal patterns of consciousness).
To go back to my story, I could have used magick at that time to force things up (fortunately I didn't do it because my intuition sent me a big no-no) but that would have been violation/manipulation of the free-will of the other person and it would also have negatively affected me since it would have pushed me away from an important life lesson (maybe so far away that I would not have been able to find my way back).
I guess this is fine if the STS path is your choice, but it's not mine. BlackBox, I have to strongly disagree with your explainations on STS/STO. Yes, I have decided to incarnate on a STS planet in a STS body. However my path is STO and I'm sure that I decided to come here to purify my dedication toward this path. What better way to do that when everything around you is STS? So if you want to "climb the pyramid" and go STS, fine, but please don't assume that this is the case for me and others. I do not wish to have anything to do with pyramids-climbing, and despite the mistakes I have done in the past (for which I'm thankful, since I learnt important lessons), I do not wish to manipulate the free-will of others to fit with my ego's desires.
Long post, I hope I didn't go too off-topic but I felt this was important to note.