I've heard a lot of interesting ideas here that have definately expanded my spectrum of understanding. To flesh out where I'm coming from though, I'll share a few of my thoughts on the warp.To a human mind, any mind regardless of training, I believe that the warp presents an immense personal danger - the greatest danger seems to come to those who think themselves safe. If, as has been stated, the Warp is Chaos - Entropy - then it should completely repulse contact with the natural materium - which is why daemons are unstable. Thus a psyker drawing on any use of the warp is introducing a physical embodiment of 'wrongness' to the materium - things that could not and should not exist according to the (few) natural laws of order. I strongly approve of Jack of Tears notion that the psyker (if not every one else) should be aware of what exactly he has conjured up.
But the warp has always struck me as something fearful to the sane, even our favorite literary inquisitors - something you ought not to look directly upon, even while you manipulate it. Of those individuals seen to have 'fallen' or to be more privy to the nature of the warp, they seem to me to fall into one o two patterns of behavior due to 'over-exposure' to the warp: either a sensation of complete personal futility and irrelevance (such as you may get if you consider too long that, really, you are nothing but a walking bag of carbon-bonds that has no business possessing a sentience), or the exactly (rather Tzeentch-like) opposite - an awareness of the extreme -connectedness- of absolutely everything in the universe, how every mundane action, every fales preception of free-will, fits into an orchestrated, entropic plan - that of course, comes full circle back to futility. It really is mind-boggling. But, when one draws upon the warp, I try to invoke those notions, and the invocation is usually dark-seeming.