Thanks for the question. I really don't think there's much if any advice i can give you. My real expertise is in the area of frequent flyer miles and points and I'm not a professional when it comes to developing online forums. I can five you a link to a Web site that helps owners of these types of forums with advice. It's called the AdMin Zone and here's the link:
http://www.theadminzone.com/index.php

While you may see FlyerTalk as an example, there have been several if not dozens of attempts to replicate it with similar Web sites and you'd have to ask the owners of those Web sites if they felt they had been or are successful. All I know is that I'm a very lucky guy. Lucky in the sense that FlyerTalk needed no admin nor moderators when it started. The sole attribute of its success has always been directly from its members and we were fortunate enough to be in the right place at the right time with absolutely the best combination of extremely intelligent travelers and their passion for the topic that FlyerTalk has become famous for.

With all that luck on my side - and the original members of FlyerTalk it's likely I really do not know enough to actually be of any help and perhaps you can find as I have from time-to-time, some good information and advice on the Web site I noted for you above.

In closing, I might suggest a different idea for you. Unless you really are building a travel Web site with high ambitions as a business, I might suggest you just be happy looking over a couple of Web sites with topic interests along the lines of what you are thinking and simply just become a good member of that Web site. It's a lot easier and may be a lot more fun for you. Earning respect from your fellow travelers is a reward of its own. But as I noted, that's only if you are not considering the Web site as a commercial interest. I find there are plenty of existing Web sites and at the end of the day, only you can answer the question: Does the world really need another travel Web site? If the answer is yes as others have started some over the last few years, it's because somewhere they have money (often someone else's) burning a hole in their pocket.

Hope this helps and good luck.