Your core concept
Obviously, you're setting up a website for a reason. Although that reason could change many times before you are done, your site must focus primarily on one core concept to be successful. This does not mean that your site can't, or shouldn't, offer other types of content for your visitors, but you need to keep one primary focus which everything else can branch out from.
For example, let's say you want to start a website about raising parakeets. You have a lot of firsthand knowledge and success in this area, and have decided to share that with others that could benefit from it. So your core concept is to provide information about raising parakeets. Simple, eh? Then you start to think about other aspects of parakeets that your visitors might be interested in, like training them, their diets, health, etc. Those would be valuable areas to include, as they all are related to raising parakeets, and would more than likely add additional interest for your site visitors.
Since that worked so well, you decide that maybe you can expand a bit. Perhaps you want to add content that talks about your history with parakeets, how you got interested in them, etc. Maybe the history of parakeets themselves. Even information on the best types of cages for them and how to raise them in groups. This is ok, since this still all revolves around the core concept. Then you start thinking about how well your parakeets get along with your dog Max and your 2 cats, Lumpy and Bumpy, and maybe add a few pictures of them together. It's so cute, in fact, that you decide to give max, Lumpy and Bumpy their own pages on your site, since they are related to your parakeet in some way. After awhile, a few photos of your family, your last vacation, even Thanksgiving dinner start popping up on your site. And after awhile, your visitors aren't really interested in your site anymore and stop visiting. What went wrong?
The problem here is that you've gotten away from your core concept of raising parakeets. Although Uncle Bob getting tipsy and falling face first into the dressing at Thanksgiving may be humorous, it has nothing to do with your concept, although in your mind you played connect the dots to make them related. The main idea here is to know, understand and stay true to the concept of what you're doing. With few exceptions, and excluding some types of websites, you won't benefit yourself by drifting away from your core concept.