There's an easy and a hard answer. The easy answer is what's enforced ruthlessly right now vs. what's been enforced ruthlessly over the past two decades. I think I have at least one answer to the former question (don't touch Social Security). My only answer for the latter would be abortion rights.
I don't think the abortion situation is analogous at all. It's true that fealty to a pretty strong form of the pro-choice orthodoxy is considered absolutely vital to being a Democratic presidential nominee, but below that level there's considerable tolerance for deviation from the reproductive freedom line. I understand why pro-life activists aren't Harry Reid enthusiasts, but the man is the Majority Leader of the US Senate, and he's certainly pro-life enough to call into question the idea that the pro-choice line is "ruthlessly enforced" inside the progressive coalition. David Bonior served as the pro-life number two House Democrat for over a decade and now plays a prominent role in John Edwards' campaign. And beyond that, the view that pro-choice groups hold too much sway over the Democratic Party is something that's regularly voiced by liberal writers and in liberal publications without them getting run out of town.
Social Security, in recent years, seems much closer to the mark since it seems to me that there are a number of people who favored some form of Social Security privatization in the 1990s but who were basically frightened (a good thing, it seems to me) out of saying so in 2005 out of fear that going off-message would severely damage their chances of playing a role in Democratic politics. That, though, is a phenomenon of pretty recent origin and it's not totally clear whether or not it will hold up.