The importance of design on the web is one of the most "critical" aspects of a start up's success. You get a lot of people doting on new websites for their beautiful landing pages and along with it designers excited to profess their obsession with "pixel perfect" UI. In fact, the importance of UI is extremely overblown. The way pages connect and function and where users interact is what will make or break a web service, not the UI.
I'm going to go ahead and use the analogy of a house as a website and users as prospective homeowners. I think it works.
You need a welcoming entrance. It should lead to the most impressive part of the house or perhaps a general tour. It should make visitors say something like "Wow. This seems like a nice house. I'm going to go snoop around."
Someone has to put food on the table. On twitter the food is a tweet, on ebay the item for sale, on youtube the video. Whatever the meal, the chef needs the stove and oven in order to put out their best food, and for them to be able to cook day after day. If the kitchen is especially well built, chefs can socialize with each other.
The Dining Room
There should be an airy communal space where everyone who's in the house can interact. The food is served and the hungry guests chow down. They get the chance to like, buy, or retweet the food so the chefs know they liked it.
Sometimes you need to take care of business, readjust the settings if you will. The bathroom doesn't have to be fancy, but it does need to be 100% functional. Otherwise things get messy and you spend more time in the bathroom than you should. That never is a good experience.
The bedroom is where an individual can come, own their space, hang a few pictures, and make sure they're presentable to the world. This room needs to be functional, under the user's control, and also fun and homey.
After investigating the various rooms the user makes the call: should we move in? should we spend time here? is this where we want to cook and eat our meals? They're snooping around deciding if this is the place where they want to invest time and energy. This is where they'll tell their friends and family they're living. If the floor plan has missing, uncomfortable, or hard to find rooms, users will look for another place to live.
Only after the lease is signed do prospective home owners discuss furniture and paint color. UI is furniture and paint color. Hacker news, digg, google, twitter, facebook. They all made sure their floor plans worked great before worrying too much about the furniture.
Make sure what you're building is somewhere livable before you buy furniture. It just doesn't matter how nice the living room couch is if you have to walk through the bathroom to get to the kitchen.