I was over at RailsRumble this morning, reviewing entries and casting my votes, and I noticed a few things that set some of the sites head and shoulders above the rest: previews, test accounts, and screencasts.
Many of the sites simply had splash pages that said "log in or register" to see what the site did. In most cases, I didn't bother. There's nearly one-hundred entries in the competition; I'm not going to bother creating that many bogus accounts. Kudos to the sites that support OpenID, but Dr. Nic already covered that.
Some of the sites with really slick designs or very clear and concise explanatory text coaxed me in to signing up. But in most cases it was a crap shoot, especially the ones that expected me to read paragraphs of exposition.
Sites with screencasts were immediate favorites. It doesn't get any simpler! A screencast shows me what the site has to offer in a quick and easily digestible format. The creator of the site knows best how to use it to its full potential, and they can explain it all in full motion video while talking me through it. If I were President of the Interwebs, I would make it a law that all sites have screencasts.
Previews in the form of screenshots with annotations are nice, and I highly recommend them as second-best to a screencast, but they require me to read, interpret, and study the presentation. That's work. The less work I have to do to learn what a site can offer me, the quicker I'm going to get to using it.
Test accounts are a double-edged sword. The cons are that once I'm in I have to explore; I have to wander around aimlessly playing with the site, trying to figure out what it has to offer. It also seems that with at least one of the sites, they didn't disable the "change your password" for the test accounts, as I was unable to get into them using the credentials advertised on the front page. However, for those of us, and those particular services, where you really need to "try before you buy", test accounts are golden.