1) A unique idea - don't copy an idea
2) A good short and well balanced Domain, two words together is the max, best is one word
3) Business plan - plan it all out - were do you want to be in 10 years time - what users are you looking for - how much income will you need - were do you upgrade your server - what scri[ts to use - what board software if needed etc etc. plan it all
4) Time - Starting a new webpage takes time - as I say: its not how long you can run but how far - at the beginning nobody will know your page, and you will have to "fake" conversations (comments etc.) on your page to make it appear busy.
5) Quality content - you NEED quality content - stuff that you wouldn't see some other place - for that you need to build up inteligence and a lot of manhours, took me ages to find people that search for my webpage for all the newest military videos from afghaisatn and iraq.
6) Money - A good webpage will cost money and if you are not prepared to make at least a small investment you can give up right there.
7) Use scripts and learn to use scripts that are free of charge don't buy the most buffed up and expensive software - I started Apacheclips.com with a free videohosting script until the page got to busy and then upgraded to the best that is out there, its all about testing if your idea has any potential and then invest money.
Be prepared that you will make a loss for a while - Busy webpages cost money, there is no way around that - and there is very little chance you get any income until you have a good userbase and a lot of guests viewing your webpage.
9) Dont save money on the server, if you do you pay for some slack administration (unless you know how to look after your box) and your page will get hacked defaced and raped - start with a shared account and keep looking for the next which should be a VPN server and then maybe a semi dedicated and than a fully dedicated server.
10) Think Big... Facebook started small one day too - now they need 30.000 servers - and they are still growing - chances are very high that you will start a new facebook or twitter craze, but having a small succesful page with members that care about "their" page is a lot of fun too.
Any questions welcome