This guy is the strangest character I've seen in a long time.
1. Free is a hard price to argue with.
2. It's easy to understand. Everything is in front of you.
3. Because it's free, it's easy to see exactly what you're getting.
4. Craigslist is also free, but it's restricted to specific cities. It's true that the list of cities has grown massively, but the fact that Craigslist is chopped up into subsites makes it feel smaller - especially if your city happens to be the one with only 2 ads on it. Plentyoffish keeps the geographical partitioning in the background, and sustains the illusion that the entire world is one big room.
And let's face it - Plentyoffish may have somewhat primitive design, but it's like fine art compared to Craigslist, which doesn't even give you thumbnails to look at.
5. It feels like something one of your friends might throw together in a weekend - and that's good, because the result is unintimidating. There are lots of people looking for dates who cannot write or take decent photos, and the last thing you want to do is scare them out of posting by setting high design standards. You're not designing an art museum. You're designing a very fancy bathroom wall for people to write on. Plentyoffish nails this aesthetic - it feels more like a backyard BBQ, or a friend's untidy family room, than like a fancy cocktail party or a business meeting with suits and ties.
Which suggests that there might be room in the market for an upscale Plentyoffish - for people who want to look for dates in the online equivalent of a country club instead of at a bar. Perhaps that's what the subscription dating sites are for?
Any more guesses from the audience?