Bob's right. Those kinds of sites have been done, and done, and done. You'd have to not just set the site up to have ads (and I would have ads for computer gadgets, design templates, domains, SSL certificates, billing software, merchant accounts...things that someone shopping for hosting might want, but *not* ads for other web hosts), but the reviews would have to be good quality, AND the site would have to be known as being completely and utterly impartial. No top ten lists, no "best web hosts", nothing that a web host (like Lunarpages or GatorHost, to name some biggies) could conceivably weight their own business' exposure. Otherwise, many hosts - myself included - wouldn't really care to sign up for such a directory. And if you don't have content, you won't have many visitors, so you may not be able to continue getting ad revenue.

Get a site going that's utterly neutral, and you may have a shot.