Being holy, that's what it's all about. So why does G-d feel the need to command us specifically to be holy?
The is a highly popularized explanation of the Ramban which basically goes like this:
Yeah, mitzvos are to make us holy, but not all of life is so clear-cut. There are many details in life that are not covered by specific "do this, do that" guidelines (how much to eat, how much to spend on a car etc...) and our job is to sanctify ourselves in those areas using our own boundaries. And on the other hand, it's possible that a person can earn the dubious title naval b'rshus haTorah: someone who follows all the specific guidelines mentioned in the Torah, but is a completely unrestrained in the non-controlled areas.
Maybe you are thinking: well, maybe Ramban only meant this as a drash. However, I have seen (albeit in only one context) that R' Moshe Feinstein uses this Ramban as one of the reasons that smoking marijuana (or hashish, as he refers to it) is assur; it is an indulgent activity which flies in the face of Jewish values.
To me most interesting part of this concept is that it allows for subjective areas in halacha. So many of us were taught that "the beauty of halacha is that it touches all aspects of our lives: how we eat, get dressed, talk etc... and it tells us the proper way to serve G-d in every way possible," or something to that effect. But apparently, this isn't completely true; although the mitzvah of kedoshim t'hiyu does tell us that we should be aware of Jewish values in everything that we do, it does not always tell us what is right and wrong.
How can the Torah leave it up to us? Why should we have to ask ourselves what G-d would want us to do? What is gained?
I have two approaches, and I think both are correct:
1) G-d, better than anyone, understands the fact that we need to feel like individuals and we need to enjoy life. He also knows that every person finds meaning differently. So, in order to strike a balance between the control of the Torah and necessity of individuality, G-d allowed certain aspects of our lives to be regulated by us.
2) There are the commandments of Torah and there are Torah values. In order to impress upon us the significance of Torah values, certain areas omitted from the list of "do's and dont's." By leaving thing out the Torah forces us to to make our own decisions, decisions that are meaningful to us and based on Torah values.
There's the d'var Torah. Now, let's get political, liberally and conservatively:
Unfortunately, the significance of subjectivity and the beauty of individuality has been lost on much of our generation. How many people out there think that there is a specific way for a "Ben Torah" to dress and spend his free time? Lots. How many people out there think that a "Bas Yisroel"(I purposely replaced the 'a' of yisrael with an 'o') has to stay away from certain professions and has to to look for specific characteristics in a shidduch? And unfortunately, this close-mindedness has reached the point of masquerading as Halacha Moshe MiSinai.
This doesn't mean that people shouldn't strive for growth. However, every person is on a different level, and to impose your way of life on others is, I believe, contrary to the message of Kedoshim T'hiyu.
The other side of this message is that we must govern our lives within the confines of the Torah, even in areas not discussed by halacha. It is often difficult to decide what is right and wrong in these areas and even harder to follows those decisions, but that doesn't remove our responsibility. However, for many Jews, this concept isn't even on the radar. Torah values include modesty and humility, two values that many in our generation ignore. Affluence and immodesty are pretty common in many of our communities, perhaps, due to the absence of the message of kedoshim t'hiyu.
As always, we must be honest with ourselves and find our own balance...