In terms of how to live our lives, the Torah gives concrete answers that we can apply. The Torah is, in my opinion, the essential aspect of Judaism. Reform Jews try to minimize the Torah's importance. I think that they are really missing the point of what Judaism is.
However, there is little in Judaism that defines the mysteries of what G-d is. We know what God has done (in relation to mankind), we know His relationship to Jews and Christians in particular, but we don't know very much about what G-d is. And I don't think that we can ever know this, at least not on this temporal plane. I can accept the fact that G-d transcends understanding. But I would like to offer a metaphor as to why I think we are incapable of understanding.
Imagine a two-dimensional creature. It lives on a sheet of paper. It can see side to side but it cannot see up. To this creature, there is no such thing as up. It invents tools to discover how old this piece of paper is that he is living on. He creates telescopes so that he can see to the edges of the piece of paper. He understands all the science that exists so that he can understand the nature of his paper and the other creatures that share the piece of paper with him. He is given and establishes laws that allow him to live in harmony with others on his piece of paper. And ulitmately, he struggles to make his piece of paper a better piece of paper, at least better than when he was born into it.
But he cannot look up (or down). There is no such concept to him. But the reality is this: Beyond this piece of paper, there is a whole extra dimension that does exist. What does this extra dimension look like? I cannot say, because I happen to be living on a piece of paper, and I call it "the universe". No matter how far I develop, I will never be able to conceive of anything beyond that.
That is the only way that I can conceptualize G-d; He transcends my universe and is beyond what I am capable of experiencing or understanding. Some of us have the sense to realize that there is something beyond that which we can conceive. Although we don't know it's nature, we know it is there. Unfortunately, there are some who do not or cannot understand this. They are limited to their piece of paper and I feel sorry for them.
The mysteries of G-d are beyond human understanding. I accept that as a person who cannot physically transcend his plane. But that is all the more reason to ponder G-d and contemplate now and then, but more importantly, why we should follow His rules-the Torah. I like the fact that we can still do what Abraham did 3,500 years ago. We can look up at the stars and know.