I find all of this very intriguing. Assuming that both Craigslist and PlentyOfFish are successful for the same underlying reasons, it's worth pointing out similarities between the two. Note that while I've been using Craigslist for years, I have very little real experience with PlentyOfFish, so feel free to correct any invalid assumptions I make:

1. Plain user interfaces. The choices of fonts and colors seems very personal, non-corporate. There's nothing sleek, modern, trendy, creative, or flashy about either of these sites. That's not to say they're particularly simple or efficient.

2. Unchanging user interfaces. When you go to Craigslist, at least, you can be sure to find the exact same site you were using one, two, three years ago. No surprises, no re-learning, and no feeling left-behind.

3. Initial focus on small community, growth through word of mouth. Craigslist had San Francisco, and PoF had Vancouver I believe? There's something to be said about making users feel like they are party of a small, exclusive community. (Look at the beginnings of Facebook.)

4. Content created almost entirely by users. Craigslist does little more than categorize users' posts, and PoF is, well, a dating site.

5. Open: you don't have to know someone or get them to accept a friend request in order to interact with them. Once again, these sites make connecting with others easy and direct. (Although the signup process at PoF is understandably more involved.) When I go to Craigslist Boston I really do feel like the site represents the entire community.

6. Pretty much free.

I'm sure I could add more, but the recurrent theme seems to be getting out of the way and keeping the focus on the people. Neither of these sites rely on revolutionary tools or features or algorithms, and neither seems particularly interested in impressing the tech community.