This is pretty subjective and I think the key is the questions, "How do they get to my site?" and "Who is your audience?".
If people are going to see your website in commercials or other non-web based mediums then you probably are targeting a broad enough audience that adding the www. and .com to your domain would reduce confusion and allow more users to get to your site.
On the other hand if your domain is something that is mostly going to be advertised online (like del.icio.us) then having the www. and .com is much less important because people will be clicking on links rather than typing the domain in.
Furthermore, your audience matter because if you are lobbying to non-technical people then you have the issue of how do 2 friends verbally communicate the site. For instance if 2 middle age men talk about ebay next to the water cooler it is easy enough for them to communicate the site. But if they are talking about youtube they could have an issue. Case in point, the website www.utube.com gets thousands of hits daily from people looking for youtube.com.
Finally, the point on the www. should be moot because you should just have the www. redirect to the one with out it or vice-verse. The real concern should be about using unusual domain ender (anything but .org, .com, and .net are somewhat confusing in my opinion) and using website names that are phonetically similar or the same as another site, like youtube and utube.