I agree that it would be awesome to be able to filter listings on Craiglist through a neat-o UI mechanism ("Hipmunk for apartments maybe").
In fact, I basically spent the entire time I was using the site thinking "if only more landlords indicated whether the apartment is pet-friendly...". I ended up calling a ton of landlords just to find out whether they allowed pets, on the off chance that some of those who had listed no pet policy actually allowed them.
But is that really a UI problem? For me, Craigslist has already provided the UI tools I needed (you can search by pet-friendliness). The problem was that the landlords didn't include that information in their listings in the first place.
Craigslist is a site for "classified ads". Traditional classifieds (in newspapers) were usually very short, just a few words. Of course far more is possible with Craigslist, but a larger ad also requires more effort on the part of the seller.
As it is now, incomplete information has a cost for sellers (people skip their ads), but they can choose to bear that cost if they wish, presumably after comparing it against the cost of providing more complete information. Part of the reason people use Craigslist is that it is more or less a willy-nilly market that is friendly to small sellers.
So what would Craigslist have to do to make a new UI worthwhile (for me)? Enforce data completeness on the sellers. But that would turn people off. Requiring additional bookkeeping or punishing sellers for "incomplete" listings just to make a snappy UI practical would harm the community and the site.