I've done something similar with my country with 4-5 specific niches in large populated areas (and expanding as we speak) and it works great and seeing that it got even better than I expected I intend to do this with all the businesses available. I know I'm getting greedy but it really is that simple, I've been doing this for 5 months now and am going to give you a few tips to get started and to succeed and also bypass the "oh, no one else is doing this, so I won't either" problem

Here's what you have to do:
1.Get a domain,hosting and a nice looking theme(I got it from warez sites and later bought it as I had money to invest) that is specific to your niche. Also buy an easy to remember name, don't just go after exact match.
I've done this since I haven't done SEO in my site but used another form of advertising(see below).
2. Call a few businesses and tell them they have a chance to get more customers by adding their business in your directory for FREE... Most will immediately jump in but will also say what's the catch? Well you say to them that only the 1st month will be free so they can see what value my directory has, get at least 20 businesses in your site, trust me it's very easy to convince them, if you've got a chance to meet the owners in person do it, if not call them up.
3. After you have a few businesses listed call up all the others in the area. You now have something to show to them, when they see their competitors are listed they'll not miss the chance
Make them pay directly or after 1 month, your choise...
4. Now how are you going to get traffic? By advertising your directory in:
*Search engines - If you know SEO, then by all means go for it, I didn't spend much time getting my site in #1 in google(did it only with on-site SEO and no backlinks) because in my country there is not any competition at all and not too many people search the internet as well.
*Newspapers - This is what I did, advertise in local newspapers, it costs but your revenue is going to cover up your expenses plus your site will be viewed by hundreds of thousands who read the newspaper every day.
*Facebook - I implemented this recently and had great success, make a fan page/group/events and advertise your directory to FB fans, this is a whole new topic I can write about, but I won't here because this post is already too long lol
*Flyers and postcards - I have a very cheap supplier in my area where I design ~5k flyers for $100 and then pay a few $ to kids to distribute them in high traffic areas, not much targeted but still I get visitors this way.
Some other tips:
*Split test everything, I use tracking tools to track where my traffic is coming from and what advertising method is more profitable
*Start small and then get big, don't be greedy, it's normal to start seeing income after 2-3 months and it's worth it
*Upsells - This is my highest moneymaker to date. As I said internet is not so popular in my country so I offer website/mobile sites design to businesses, creation and maintainence of a FB Fan Page, Professional video creation,etc..
*I only charge $10/month(increased it to $20 to 'cover hosting fees' with the website design upsell later)
Overall without revealing the exact number, I earn ~13k(including advertising,hosting and other fees) and growing each day.
So if you want to step in to this kind business,
don't think twice and by all means do it
The figures and results might vary because of country and other factors but still I guarantee you'll have incredible results, try it and let me know.
P.S. It was a long post so sorry for any typos or mistakes in sentences, hope I helped you somehow and have a nice day!