Some critics say that there are many definitions of libertarianism -- other critics caricature libertarianism and compare a libertarian society to Somalia. There has been much written by libertarian thinkers through the years, but I doubt the critics have read the literature, and this is the problem.
I have been in many debates this past year and the common problem is that most of the people criticizing libertarianism haven't read Rothbard, Friedman, Hayek, Nozick, Chodorov, Narverson or any of the other libertarian writers. This would be like critizing Marxism without ever studying Marxism. It's not very helpful to make up a popular version of libertarianism as a strawman to tear down -- read the literature if you want to know what you're criticizing or considering -- don't accept popular reductions reduced by people who are defendng a statist, non-libertarian approach.
Yes, libertarian writers have different ideas, just as modern liberals or conservatives have different takes on their political philosophy or pragmatic non-philosophy, but there are main principles that distinguish libetarian thought from those who support different degrees of statism. It's not a matter of reducing libertarianism down to a party platform, although the Libertarian Party has one, it's a matter of understanding the principles involved and whether one believes these principles should be applied to governance or not -- if one believes the principles shouldn't be applied, it would help if they knew and understood the principles and then gave good reasons why they shouldn't be applied.