I always hear that it's execution that matters, not the idea, and that you can have a great idea, team, resources but still good execution is not guaranteed. How can that happen? I have been working on my idea to build a web application, but hearing about execution and how it differs from people to people.
It means getting things done, take the idea and make something out of it, if you don't have something to show to your potential clients, a website, a web application, service, etc... then what you have is just vaporware.
For example, you mentioned your idea is to build a web application, the design, the business model, the domain name, advertising, etc... all of that is execution and your success will depend on how well you do it - IMHO.
Good luck with your venture!
My understainding is the following:
So each business has some operations, a business plan, business objectives and an understanding of their market. You can have all of that correct, but if not consistently orchestrated, what gets done isn't consistent with the market's expectations and so the result is poor.
Unfortunately, there are so many.
In addition to the ones mentioned by John, I'd add "lack of outside validation" - where your initial idea goes straight into production, without any measurable outside feedback from customers (prospective or otherwise) and believing that people will sign up 'cuz you were so smart to think about this untapped market.
Stems from the mistaken belief that our ideas / insights are fact, and not simply hypotheses (guesses) that need testing to learn how close to fact we really are.
I would like to add something I call "mental energy", it probably has many names. This is actually obtaining the skills needed to implement something. I believe that there is a limit to the number of things you can ask others to do (even if you could afford it). Some things you just need to learn and solve yourself, even if you do not have the skills or education (maybe the education does not even exist because you have a radical innovation).
By definition a bad idea cannot be executed. There are many degrees on the bad to good spectrum. Part of great execution is knowing when to change your idea.