In some ways it reminds me of a left wing version of Free Republic, but with more celebrities, more humor and somewhat better overall production values. That said, the overall design of the site isn't that exciting. It's very straightforward and newsletter-like. The simplicity of this format makes it relatively accessible, but it's not exactly a feast for the eyes. I will say this, though - it has one extraordinarily neat feature that I haven't seen in use anywhere else, though I should have. The news section on the right-hand side of the main page actually refreshes itself periodically as new news arrives, without requiring the viewer to reload the page. Not a super advanced web design trick, but one which I've never seen used on a blog or news site before, and it helps create a real feeling of immediacy.
I think the long term viability of The Huffington Post will largely depend on whether they can keep the contributors producing at anything like the level they have in the first couple of days while increasing the quality and substance of the submissions. I get the feeling that it took some weeks to gather all the material the site debuted with, and that future contributions by celebrities are likely to come in dribs and drabs, and that the overall freshness of the material will be hard to maintain. It's also a problem - characteristic of many left wing blogs - that they don't allow any kind of comment or response from readers even on their 'The Blog' section. If the New York Times and CNN can feature user response areas on their web outlets, it seems bizarre for a news and opinion vehicle specifically designed for the web to shut out user response. That suggests an elitist attitude and an unfamiliarity with the medium which may alienate some potential readers.