First, some background. When you apply for a US visa, the officer is required to presume that you intend to become an immigrant, and it is up to you to convince him or her otherwise, that you have sufficient ties to your home country that you will return home as planned. We have a number of answers in our archives on how to show these ties, and I recommend you review them (for instance, this answer on bank statements, while talking about the UK, is quite helpful).
So the main issue your application runs into is that it is harder to show ties to your home country when you are asking to take an extended trip to the US. If you're able to spend so much time in a foreign country, you likely don't need to return to a job and don't have anyone at home relying on you. Therefore, it will be hard to convince the officer that you won't overstay your visa or work illegally in the United States. You should have a coherent explanation for your plans and the financial documentation to back them up.
Many websites give you an idea of what kinds of questions are typically asked at visa interviews. In your case, I would expect scrutiny as to why you want to spend so much time in the US, what you plan to do while you're there, how your personal circumstances give you so much free time, and your financial situation. Remember that US visa interviews only last a few minutes, so rambling or unclear answers will hurt your chances. Over-rehearsed answers may sound forced, and worse, false answers or documents could lead to much worse consequences, so always tell the truth.
Also note, as mts points out in the comments, that if you are denied the visa, you will be ineligible to use the VWP in the future and will need to apply again for a visa to travel to the US.
The main benefit a B-2 visa provides over the VWP is the longer duration of stay and the ability to apply to extend your stay or change your status while you're in the US. Many B-2 visas are issued as 10 year multiple entry visas, which give you greater flexibility, though admission to the US is always subject to the discretion of the officials at the border. There are also some legal differences in terms of your rights and status with the immigration authorities.