I think there's more to it than that. I know people for whom craigslist is one of the very few websites they interact with. It's a very different demographic from the one that hangs on HN. I think this is a big factor in the CL design and the reluctance to change it.
I also think the network effect is being overstated in this instance. eBay has much stronger lock-in because I can't list my item in more than one auction at a time. But I can certainly list my apartment in more than one service.
I already check the local paper in addition to CL when looking for an apartment. If I'm looking for a bargain, I'm fine with shopping around.
If you don't think Craigslist has an insanely strong network effect, you haven't been thinking about this long/hard enough. Why do you think it's one of the very few websites those people interact with? It's not just because it's simple (though that helps a lot). It's that a) everyone uses it and b) it has everything you could want. It means you don't have to remember 50 random websites, one for each vertical.
I may be in the minority in Silicon Valley, but I'm honestly a really big fan of Craigslist, even despite their C&D'ing me, and I don't think it could or should be "disrupted". I think it's perfect for most of the things on there: lightning fast, few restrictions, community driven. I think it's massively better than that other alternative of having a ton of small websites that each do one thing well, but differently. I shop for new stuff almost exclusively via Amazon for the same reason that I shop exclusively on CL for used stuff.
I think CL works because to most end users, it's really no different than newspaper classifieds. Classified users expect to have to wade through a bunch of useless stuff to find the diamonds in the rough.
I also think the network effect is being overstated in this instance. eBay has much stronger lock-in because I can't list my item in more than one auction at a time. But I can certainly list my apartment in more than one service.
I already check the local paper in addition to CL when looking for an apartment. If I'm looking for a bargain, I'm fine with shopping around.