Great post by Dead2.0: 11 Suggestions For Not Being a Dot-Bomb 2.0.
I like especially #3 “ Don't rely on bloggers alone“. If your business model only relies on bloggers spending money or you need to have a blog (or something similar) to be able to use your service, then your target audience is way too small. Even if blogger-numbers will explode in the future (which they should since two? three? years :) ).
And I'd like to add “ _don't rely on people having to install _anything__ “. If users have to install anything to be able to use your service, you already lost IMHO. Most people won't (or don't dare to) install a firefox extension, a toolbar, plugin, whatever… Not to speak of your upgrade hell you'll always run into if you rely on client-side installations.
And last but not least “ Make your system scalable“. Business-wise, as well as technological-wise. Start small, add features later, don't try to do everything from the start, but make your systems scalable from the start, so that if your service is getting really used, you don't have to rethink over and rewrite everything. Business-wise this means that your employee-per-user ratio should fall with time and technological-wise you basically just should be able to throw more hardware at it to support more users. Very simplified, of course :)