I had a psychology professor in college who used to say science is filled with puzzles of two distinct kinds: problems and mysteries. Problems are those questions that fill up most of a scientist's day-to-day life, and most scientists' entire lives, but mysteries are the sorts of things that require revolutionary theories and ideas. For instance, before Newton came along with his theory of gravity, the orbits of the planets and the pull of the earth were mysteries (notwithstanding some people's continuing state of ignorance). Before Darwin, the complexity of life was a mystery, something not only that science had no explanation for, but that science seemed incapable of ever addressing. The mysteries of the universe have long been the sanctuary of religious and philosophical beliefs; wherever science stops, religion has typically taken over, but the march of science into the domain of the traditionally religious has been constant and unrelenting, leaving less and less room for the mystical and supernatural.
One of the last and biggest mysteries—for me at least—is consciousness. A different psychology professor of mine from college is famous (I use the term loosely) for his contention that humans are "intuitive dualists," that we cling naturally and innately to the idea that our minds and bodies are separate entities. Though I agree with him entirely in asserting the opposite with great confidence, he stops short of conceding that science, like any religion, is hopelessly impotent when it comes to explaining consciousness; ie how a universe of purely physical entities can give rise to subjectivity. I fear it may be something that science will never explain within a materialistic framework, and to me, that's kind of a big deal.
Consider this thought experiment. First, assume Paul Bloom—and science more generally—is right that the universe consists only of physical things, and no non-physical souls or spirits or whatever. Then imagine the universe is exactly like the one we currently live in, with one major exception: you and Paul Bloom (first famous person I thought of) have switched consciousnesses. By that I don't mean you've switched bodies, or brains, or anything like that; each of your bodies and brains remains identical, but you've switched places, so that now you are living the life of Paul Bloom exactly as he is currently living the life of Paul Bloom. You're experiencing it now because it's you. Don't be tempted to object that his life would have been completely different if you were living it, since you would have done things differently; remember you've inherited, along with his body, his brain and all his characteristics.
At this point, in addition to being creeped out, you're probably fishing for reasons this thought experiment is entirely unreasonable, but hopefully you're starting to see the point—what ties your experience to your particular body?—and the implications. If you're still with me you'll have some sense that this imaginary universe isn't identical to the actual one we're living in now. It's identical for everyone else, but for you, there's a major defining difference, since you're living a completely different life. But at the same time there's no physical difference between the imaginary universe and the real one, and thus, nothing to distinguish them; according to our initial assumption that the universe is entirely physical, there is no difference at all. So how could two identical worlds be so different for you?
Point is, in a world of physical particles with no perspective, there doesn't seem to be any room for a collection of particles to form a perspective, since everything is made of the same particles, and should therefore share everything else's perspective. Everything should be completely objective, like the physical laws that make up the universe in the first place.
Next up, more philosophy of mind and another mystery: mental causation. Science, get crackin' and good luck.