From the first chapter of Eric Alterman's new book:
One reason liberals today find themselves vulnerable to vituperation from so many quarters simultaneously is the difficulty they face in explaining, even in the most rudimentary terms, their basic philosophical beliefs. While contemporary conservatives may actually ignore their own principles in practice, they can at least explain them.
It is a good point. Liberals do have a hard time defining their belief system in comparison to conservatives. Ann takes a good stab at it here with her list of things that she wants.
I think part of the difficulty is that liberals have a much more complex and nuanced system than the simplistic and naive belief system of many conservatives. But Alterman hits on what I think is the key difference a little later in the book when he notes that liberals, unlike conservatives, are mostly concerned with outcomes, not the means. Conservatives, on the other hand, are all about the means. For rightwing ideologues, the means are all important. Anti-government, anti-tax, pro-big business, privatization, etc. It is that way or nothing and it doesn't matter whether it works better or not.
For liberals, however, we aren't so hung up on the means. We want results. We want a fair, equitable and just society where people have access to healthcare and education and a clean environment and so on. If we can achieve that through small government, low taxes and privatization then that's great! The problem is, as we've seen for the past eight years, it just doesn't work that way. You need government to do a lot of these jobs because it just isn't practical for private, profit-oriented businesses to do them. So you end up with liberals supporting big government programs not because they love big government, but because it is the only means to achieve the things we want and need in America.
Meanwhile, conservative stubbornly stick to their means because it is the core of their ideology and if it does not produce the desired results then they just make excuses about how we did not stick to the ideology fervently enough.