Nothing will have a greater effect on how much traffic and income potential your site has then what topic you choose for your site. Sites that deal with a more general or more mainstream topic have more potential for traffic, and sites that deal with more niche topics will have a lower traffic potential. For example a video game cheat code site has a very high traffic potential, simply because there are lots of teenage boys on the Internet. However a site about knitting does not have as high a traffic potential because it's just not as popular. However, often the lower traffic niches can end up paying the most in advertising revenue, so in a way things can even out.
This doesn't mean that you should think of a mainstream topic for your site, on the contrary, you could still do very well with a niche site. Mainstream sites have more competition in the search engines and directories, this will make your job of promotion harder, but the payoffs for a good listing will be greater, traffic wise, than if you ran a niche site. On the other hand a niche site does not have much competition, in fact you could possibly be the only commercial content site for your niche. By commercial I mean that you will be seeking a profit. However it is an inverse relationship between competition and payoff. If you run a niche site the competition will indeed by less but the traffic for a good listing will also be less.
Also there is the issue of profitability. A niche site that gets 100,000 page views a month could be worth more than a mainstream site that gets 500,000 page views a month. Advertisers want to be able to target their ads, and mainstream sites, by their nature, have a very general demographic. On the other hand a niche site, with less traffic but a more targeted demographic, could pull in targeted advertising which pays a higher premium than untargeted advertising.
Another profitability issue also involves demographics. You need to think like an advertiser. Advertisers aren't always after teenagers or children, advertisers are after middle aged successful men and women. Advertisers are after those with purchasing power. If you goal is to make the most money as possible then remember this when planning out your site. The people you want visiting your site are those middle aged successful people, as such you'd want to target your site to those demographics. In general the more money your visitors have to spend the more money advertisers will pay in order to get them to spend it. There are some exceptions to this, and in general there are advertisers out there interested in every demographic, but the more money your visitors have the more attractive they will appear to your advertisers.
Additionally you should think about the cost of acquiring new customers for your advertisers. If your advertisers are selling video games, with a profit margin of $20, then they cannot afford to spend any more than $20 to acquire a customer or they would lose money. However if your advertisers are selling mortgage loans, boats, cars, surgery, healthcare, insurance, anything with potential profits in the thousands, then they can afford to pay a good deal for each new customer, even in the hundreds, and that translates to high paying advertising.
The final profitability issue you have to consider is buyout potential. Many times sites are built with the goal of being bought out by a larger company (for a hefty sum). To be on the radar of larger companies you need to have a fairly large site with a good market reach and so general audience sites tend to be the ones most often bought out. Keep that in mind if a buyout is your ultimate goal.
If you haven't decided on a topic for your site yet think this information over and see if it helps you make a decision. If you have already decided on a topic then use the above to help you understand the potential for such a site.
Once you have decided on a topic you need to stick with it. Do one thing, do it well. Too many businesses lose site of their core and end up failing, they'll want to offer all of these other unrelated services on their website because they have dreams of being the next big thing. Well its time for a reality check, because you will not be the next big thing, that doesn't mean you won't be successful, but you don't start out with a niche content site and end up as Yahoo.
Instead, concentrate on your niche and be the best you can be at your one specific thing. Massive portals are not what makes the Internet great, what makes the Internet great are the specialty-content sites. I always amaze myself when I think about some tidbit of irrelevant information I'm curious about and I can go online, search for it, and find it within 30 seconds. One of the first Internet success stories I remember hearing was back in either 1994 or 1995. This guy in, I think it was North Dakota, was selling Ostrich parts (meat, feathers, eggs, everything) online. You wouldn't think there was such a big demand for ostrich parts but he was pulling in six figures. That's just one example of finding your niche and being the best at it. Sure his is an e-commerce site and this guide is for content sites, but the principle is the same.
Even though that news feed or weather information or widget xyz may look cool and neat, don't put it on your website. In this aspect you really can take example from e-commerce companies. Most ecommerce sites don't have breaking news, they don't give you weather, everything you see is going to be geared towards you buying something, anything that isn't pushing their product is a waste of space. Well your content is your product, that's what people come to your site for, don't confuse or distract them with things that are unrelated to your content. Doing so can even hurt you in the search engines, how its hurts you will be covered in a later article, but for now suffice it to say that search engines prefer sites that remain true to their topic.
Also, sites that go into great depth on a single or small range of topics tend to more easily acquire traffic and links than sites that cover a wider range of topics. The reason is that when people link to a type of site, they want to link to the best one out there, and so being the best on just a few topics will get your more links than being second or third best on hundreds of topics. In this instance second place really is just the first loser.
So pick a topic and stick with it. Don't make a site about cars and art, if you must cover two topics make two separate sites, but don't make 1 site about two unrelated topics. If you have more than one passion that's great, because you'll be able to make more than one website. Just remember to keep things separate.