This is a good thread, I learned a lot. A hint: when you pick your niche, and do your keyword research...make sure to check the adcost/cpc $ amount...it should be over $1.00 and the good ones will be paying MUCH more than that. Then, once you find one with all the right criteria and good money per click..run with it. I would probably try to concentrate on webhosting because I have a site already on that niche... and mesothelioma because that stuff killed my dad. You see...if you enjoy it, have an interest in it. or a passion in it...you will be able to really work it hard.
It is not enough to have a high-paying niche, the geographical location of the users is equally important. Finance and health pay the most, but if you get traffic for countries like China and India, you won't make more than 10-15 cents a click.