I may have used this title (we are, after all, heading into year six on August 23rd), so I reserve the right to change it. Today I think it would be worth it to think about the slippery slope fallacy. Inspired by this post by John Holbo at the always worth reading (save for the occasional comment trolls) Crooked Timber. Here in any case is a representative paragraph:
Hypocrisy is the tribute that vice pays to virtue. And slippery slope arguments, arguments from unintended consequences and the paranoid style generally are the tribute conservatism pays to the deep appeal of progressive and liberal values. They are all attempts to outflank all that without engaging it. These are methods for getting off the hook of saying there’s something wrong with what liberals/progressives want. You pretend your opponent isn’t really a liberal/progressive but some secret radical. That’s method one. You pretend the results of liberal/progressive policies wouldn’t be truly liberal/progressive (because we would slip past all that or otherwise end up elsewhere than intended.) That’s method two. That’s pretty much it.
My informal sense (driven by an examination of the categories here on the left of this page) is that the real favorite of conservative types (at least the ones explicitly covered here) is that they prefer the straw man. The straw man is a fallacy of relevance, a subject-changer in other words: don't attack the opponent's strong argument, go for the weak one (and then claim to have beaten the strong one). Well that's one form of it, at least.
But this seems to me to be what Holbo has in mind above anyway. And I think he's certainly not wrong to notice the relevance issues brought about by slippery slopes. It's important, to me at least, to keep the two questions distinct–though they may overlap in the mind of the fallacy employer.
In the first place, I think the slippery slope is a variety of causal fallacy–it alleges causal series where none will likely be the case. So, if we do x (have national health care), in a few disastrous steps we'll be euthanizing the elderly. Not bloody likely, as someone might say.
In a secondary way, however, such specious causal chains make us argue against the crazy thing–systematic government euthanasia programs–rather than the actual thing (some kind of moderate national health insurance system).
This–the fact that you have two distinct logical issues–makes the fallacy hard to answer. You have to answer each part separately, but, unfortunately, by then everyone has lost interest (if they had any to begin with).