You don't want to be TOO scientific about it, because that's just boring.
All life that we know of is built primarily out of the three most abundant elements in the universe (excluding helium, which doesn't react with much). And It's not merely that these elements are by far more common than any others.
Carbon has the unique qualities of forming compounds with a long list of other elements and readily forms polymers. From Wikipedia: ". . . it resists all but the strongest oxidizers. It does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis." And it does all this at earth-normal temperatures and pressures.
Water (made out of the other two most common elements) also has some unusual properties. Its boiling point is super high, it's got crazy cohesive strength, and as a solid -- not only does its solid form take up more space than its liquid form, which is wierd, but it's actually less dense than its liquid form, so it floats.
Any chemistry based on other substances would be less robust by several orders of magnitude, assuming any plausible combination of elements exist that can form even a fraction of the many compounds we know of with hydrogen, oxygen, and carbon (I know of none).
So Alien life likely has basically the same chemistry as ours, and since it has to accomplish all the same things that terrestrial life did (photosynthesis, reproduction, etc) it's going to have found a lot of the same solutions. Especially when, in many instances, there's either no real alternative or only alternatives that are way less efficient, it's really not a stretch to assume that it would be a lot more similar to terrestrial life than we're conditioned to expect.
That's no fun. Humans with green skin or funy ears? It's been done.
More fun is something like Noodles suggestion with the piezoelectric rock monsters -- enough science to sort of sound plausible and some interesting creatures crawling around.
In theory all you need is a sufficient energy gradient and some mechanism for harvesting the energy and you potentially have some form of life.
Reproduction isn't even nexessarily a requirement: Noodles' rock monsters might be created on some hell planet when the right type of magma solidifies under a wild magnetic field that lays down the piezoelectric material in the right pattern, and they live asexually until they get destroyed somehow.
It's more important to be internally consistent within the setting and sort-of plausible than to let science kill your soul. It's probably not a cooincidence that most sciences, especially in the "hard" sciences, have little interest in science fiction.
Ultimately the science in the background doesn't do anything useful to keep players or readers or whomever engaged in the setting -- whatever you create has to do something interesting and interact in interest ways with the other denizens of the world. It only has to make enough sense to keep them occupied until they get caught up in events; if someone spots a flaw at that stage, they're apt to be too invested in things to pay any attention.