One thing I've read is that the reason it's harder to pin down what democrats stand for and why they tend to be centrists is the fundamental difference between who they are aimed at.
Republicans aim themselves at two main demographics: 1) Rich people who don't want to share their money with the poor and 2) uneducated rural white men who want to maintain the social/cultural dominance of Christian White Men. Their are some smaller groups like Mormons and some subdivisions (ie fundies vs more secular anti-PC types) but that's mostly it. So all a republican needs to do is say he'll cut taxes, ban abortions and hurt the immigrants and he's golden. To advance himself what he must do is declare he'll cut taxes even more then the other repubs, ban abortions even faster and hurt immigrants even worse.
Democrats meanwhile represent everyone else: Every non-white ethnic group, every non-christian religious group (except Mormons), Gays and Trans folk, educated whites in the cities etc. And these groups have very different priorities and beliefs. So a democrat needs to be able to appeal to a much more diverse audience. So he or she advances himself or herself by being someone who can find common ground and be acceptable to everyone, even if not what any one group would want most.
Also no offense Tol, but can you define what Neoliberalism is? It's just I kind of think the word is in the same category as fascist, Communist, Neoconservative etc of labels few people put on themselves but are frequently thrown around at anyone the thrower doesn't like.