Mine failed because I wasn't able to get enough users to signup in my website. Basically I created a website (http://www.iGanit.com) - it is exactly same as Ebay, but for services. Instead of products, people will post their projects, for example I need to get my home painted, then they would invite bids for the project, then they would go with the best bidder, it could be either the least expensive bidder or the best quality bidder (perceived). Even now it seems like such a good idea, but I didn't know how to get users to signup back then.
I focussed all my time on building the website, finally built it with whatever technology I knew, made it work.
But I didn't have any clue on how I was going to sign users up. I needed to get Service providers and Service seekers. I hired a marketing person, he recommended doing an SEO approach, he wrote a few articles and he said those articles will create a viral effect and get traffic to my website. I paid him about $3,000 without any agreement on the metric he needed to meet, and hoped for traffic, But users never came. This was in 2007 timeframe. That is when I decided I needed to focus on sales. Whatever company it is you are starting it is always about the users and sales. So even before building your product focus on users and have a strategy to get sales, that means you really need to solve a problem that many people have, and also provide the best user experience possible so that it is very easy for your users to get the resolution for their problem. And do plenty of validation for your idea, focussing on the problem you are solving, identifying the users that have that problem, then how are you going to approach those users and what is that one phrase you are going to tell them on why they need to use your website - value prop & messaging - also have a very good strategy to communicate that value prop, one good example is approaching forums where your users hang out and tell them about your website and the value prop, if you are really solving a problem users will come.
That is why it makes a lot of sense to join an incubator and get guidance on how to do this. There will be entrepreneurs that "have been there done this" will guide you and help you. If you don't have this figured out you are simply wasting your time. One incubator I know that does this very well is Technology Business Incubator | ATDC. The good thing with doing this is you know early on your idea is not good and you can pivot into something that is a good problem that you can solve, instead of realizing after a few years, technically that time is not wasted because you are always learning something but you didn't make any money, but could have been much more useful had you pivoted early on.
Also check out my book Puga Sankara's Supply Chain Blog - that is a must read before spending Hundreds & Thousands of $$$ on your MBA or Masters Degree to get the best ROI or before starting your Entrepreneurial journey.