Over about the past year or so, since I started creating multiple websites the advice has always been to create a website based on a particular niche. The theory goes that it is difficult for new players to break into highly competitive markets.
I think niches are still a good way to make money, but once you have discovered a successful niche, I think you then need to start thinking in verticals. One thing that has stuck in my mind was something which Darren of Problogger.net said in an interview. The question was if he had to start again, what would he do differently if he had to start again.
He replied that he would stick to the camera or digital camera vertical. He has run various blogs on a wide variety of topics, but it seems like his camera review site, his newly launched digital photography site and of course problogger are his main areas of of interest.
If you have one successful website it is much easier to piggyback off this website to launch a new website along a similar theme or vertical. For me the Tokyo/Japan vertical has worked well for me. From my Tokyo website, I have created a Japan job site and a Japan hostel booking site, both of which are doing nicely.