As a philosopher who (at some point in the distant past) regularly taught critical thinking courses in which "the slippery slope fallacy" was featured, I have some thoughts about the slippery slope. First, not every invocation of a slippery slope is fallacious. The rough structure of slippery slope thinking is this: If we accept A (where A is a belief or practice that in itself appears to be a fairly small change or dissention from some established system of belief or practice) then we will inevitably be led to accepting Z (where Z is a radical departure/rejection). Slippery slope thinking is NOT fallacious if one's REASONS for accepting A are reasons which also imply that one should accept Z. Slippery slope thinking IS fallacious when one's reasons for accepting A are NOT reasons which imply one should accept Z. So, for example, when it comes to the slippery slope argument that if we accept same-sex civil marriage we open the floodgates and will need to endorse polygamous marriages and interspecies marriages, the question of whether this is fallacious CANNOT be answered apart from looking at the reasons WHY same-sex civil marriage is being endorsed. If the reason is something like, "Well, people should be able to have whatever relationships float their boat recognized by the state so long as it doesn't involve overt violence to another human being," then that's what we might call a "slippery slope" reason--a reason that slicks up the slope like a stick of butter and sends you sliding into Farmer Joe marrying Myrtle the Goat. But if your reason has to do with a principled opposition to overt legal discrimination based on sexual orientation unless there is a compelling state interest served by the discrimination, then it is highly unlikely that you'll end up with Joe and Myrtle in the courthouse. With this in mind, consider those who invoke slippery slope thinking to discourage Christians from questioning elements of some established "Christian package." They argue that if you start to question any piece of the package, you are led down a slippery slope towards rejecting the whole package. This has the look of being a slippery slope FALLACY. After all, one's reasons for accepting evolution might have absolutely nothing to say in favor of atheism--in which case, there is no slippery slope to atheism here. Unless, of course, one adopts a set of beliefs ABOUT the belief-package (as PART of the belief-package) such that a challenge to one piece of it entails that the whole must be cast aside. Consider certain beliefs about the Bible: if (a) the entire thing from cover to cover is taken to be the inerrant word of God, (b) the authority of each passage is thought to depend entirely on the Bible as a whole having this status, and (c) the Christian faith is taken to depend entirely on the authority of biblical passages, then accepting an objection to a single biblical claim requires rejecting the authority of every single passage and so giving up on the Christian faith. This doctrine about the Bible and its relation to Christianity, in this case, functions to CREATE a slippery slope that would not otherwise be there. The interesting question is why a religious community would embrace such slippery-slope-generating beliefs.
I think that there is another issue at the bottom of the whole Rob Bell "love WIns" thing, and it is something that Roger Olson has touched on, and that is a super repressive hyper-neo-Calvinism versus, well almost everything else (including other strands of the Reformed tradition). Many times someone makes a statement and it is perfectly Orthodox within historical Orthodoxy but it is outside the edge of the neo-Calvinist movement so it is labeled as "heretical." As a Baptist that grew up United Methodist and am still heavily influenced by Arminism (sp?) and Wesleyan theology (Bill Leonard has called me a Wesleyan Baptist) this is a fissure that needs to be addressed.