Most of them are simple in concept, but big in opportunity. For example, an iPhone review website. The opportunity there is huge. You have tons of people who need to get more/better info about apps before they buy than the app store currently offers. The business idea is very simple. Build a good review site, promote it, most likely rely on an advertising model. Based on how quickly the iPhone market is growing, I think the users are there to make it a profitable business. But it requires time an energy. On the one hand, a good website is essential, and easy of use and security (making sure reviews aren't skewed) is also very important. In fact, a strong web framework makes for less work in the future, so developer skills are critical. On the other hand, building a business, working with advertisers, long term strategy, promotion and customer service will also be important. Its too much work for one person, its just a lot of work in general. But I think it would be a great business.
Don't even get me started on my iPhone app ideas...