A Niche you can Monetize
There's no point spending all that time creating a website about something you can't make money from. You need to be able to monetize your niche so make sure it's got products to promote and desperate buyers.
Not people who are looking to just solve a problem, people who are willing to PAY for the solution. A great way to see how profitable a niche could be is to do a search for the term on Google and check to see if there's any (and how many) Adwords ads for the term.
Adwords affiliates will only display their ads for any length of time if their making them money after all Since they pay for every click. Also, check to see how many products you could promote (if any) there are for that particular niche.
If there's just one or two, then perhaps it's got a big target audience, but they just aren't desperate enough to actually spend money.
Bear in mind that some niches will sell better than others. E.g. out of all the niches I would say that "making money online", whatever the method, is the one people are most prepared to pay for.
Side Note: When considering if a niche is profitable remember to use your common sense. Seriously, so many marketers just look at the numbers or they're following a system so strictly that even if they object to it personally they'll still go ahead. I recommend that you don't. If your common sense tells you that a niche won't be profitable, don't go ahead with it even if it's meets the other criteria. This is your business and always follow what you believe is best, remembering of course to take the advice of others at the same time. But don't follow courses to the letter, not even mine!