The Cs (in order of progression):
It goes without saying really but if a site doesn't have a unique and solid content proposition that it's going to be very hard to be sustainable. The core product is the foundation for everything else to be built on, so quality is critical.
No one is perfect. Most people aren't outstanding or capable of leading (aside from someone like Seth Godin). To produce something truly amazing out of a one person outfit is difficult. The origins of the web lay in collaboration between educational institutions. Better ideas are always generated by working with others. Google's whole ethos is collaboration and look where it's got them. Personal brands are only created on the web from collaborative brands first; you need communication with your audience before you amount to anything greater.
You can have the best site and offering in your market, but what does it amount to if people aren't talking? And I'm not just talking about the obligatory conversations happening in the comments sections in blogs or forums. Visitors to sites need to be talking about them offline. To friends, associates, family, colleagues, strangers - word-of-mouth marketing is invaluable. Conversations are important for collaboration and collaboration is needed for producing original content.
If you have conversations, regular and repeated visitors, site evangelists, new users and so forth you will have a budding community. Much has been written online about 'community' so I don't want to go into too much detail here - it's also pretty much inherently obvious.
This is a new addition to the Cs. The web is a complex ecosystem which is growing fastidiously every day. I've already touched on 'content saturation' here but it's important to note that you will always have new entrants to the online and offline markets, especially in publishing. The barriers to entry for online content production are practically nonexistent, with anyone being able to set up a blog for free and upload videos onto YouTube. Yes you need to be aware and prepared but also rational and realise that as long as you have your position down pat and are keeping your core readership happy you will be able to still exist in the food chain. News share sites and the 'mashable' kind are in a very strong position, because they don't act carnivorously. And number two, collaboration rears its head again!
I have a final C that I would like to put out there, this is more of a mantra than a must-do.
I've see it too many times - websites that are promising but upon further inspection just sound arrogant, aggressive and pretentious. You know charm is lost when you see that Wikipedia refers to the BlackBerry Charm, phone charms, lucky charms and charm bracelets before the origin of the world. Even the awful TV series gets a look in. An example... I do not want to be greeted with an 'about' me page written in the third person about your life, when you are Gen Y and haven't really achieved anything yet. Let's not forget it - afterall, charm is the biggest attractant you can have! Humble people are truly unique.
1) Content 2) Collaboration 3) Conversation 4) Community 5) Coexistence