I wouldn't say there is anything wrong with 40k, it's just been taken in directions that I dislike. I'm not averse to change but I do feel that sometime around 2nd ed decisions were made that slowly shifted the focus. I saw an interview with Rick Priestly recently and he was saying that the scope of 40k is so massive that much of it should be unknown, both historically and in the present; the Horus Heresy was the example he gave, 10,000 years ago we were in the Stone Age and beyond a few cave paintings and archeological finds we know very little about it; the Horus Heresy should be just as unknown, a mythical time where we can only hope to garner a faint glimmer of what actually happened from the scattered and mythic sources that remain.