There are many factors that a venture capitalist looks at when deciding whether to invest.
First and foremost is: Is there a real business here? A great idea doesn't always translate to a great business. For example, someone once came up with a great idea to advertise on the back of golf carts. They thought this was a great business, and perhaps it is for a very small company, but there are only so many golf carts in the world, so it can never really be a big business.
Secondly, are there competitors that are already in the business. Why would this newcomer succeed? A special piece of software, knowhow?
Who is running the business? Do they have business experience along with the idea? Can they take something that was perhaps a 1 person business to a 1000 person business? Do they have the experience?
Will others also want to invest? Quite often Venture Capitalists (VC's) want to make sure they don't take the entire risk themselves. They want to make sure other VC's see the same vision that they see.
How much money does it take to prove out the viability of the business. Will they know that there will be success if they invest $5 million dollars, or will it take $100 million to know. If it is $100 million, is it worth the risk.
The people. If the potential investment passes all the other tests, the key is in the people. Are they passionate? Are they smart? Are they capable and dedicated enough to drive the business home? Can the VC trust them? Work with them? Chemistry is very important.
If you want to go into a role like this, there is nothing more exciting. You have to spend a good deal of time though doing research, learning about how to run businesses. What works, what doesn't work. Successful VC's have been doing it for a very long time, but I believe all of them would say that it is worth the time they spent getting there.