Absolutely right on the simple.

But my favorite example - plentyoffish.com - isn't exactly "clean" IMO.

It's amazingly ugly and crowded with colors and text, but the usability remains true to user's expectations.

I think that factors into the genuine feel of it - feels like one guy did it because he wanted to help people find a love connection . . .

You're right about "clean." It doesn't have to be there. The key is definitely simple.

Google (my favorite example, although they are morphing from this in the last year or so), Craigslist, and Twitter all kept it clean and simple.

I like the idea of a site thats a little dirty but not too much so that the product seems like a deal yet the customer service is still there.