In terms of investors I would add that there are above 75% of all VC investment in the world are made in the Silicon Valley (I don't have the exact number, but it's in that range). And don't be fooled, getting VC investment without having some kind of network to introduce you or backup the quality of your work is much much harder. By being in the Silicon Valley you meet the VC folks, you meet former Entrepreneurs that were already funded and successful, you meet the crowd that you need to get build the trust you need to get your funding.
Not to say that being in Austin TX, or Boston, or else is not good, but the Silicon Valley is a very helpful place to be.
Drawback: COST OF LIVING!! it is expensive to live here and as a consequence wages are high so your startup cost are high. Also, competition for talent is very strong, so you have to pay people more (increased cost again). And finally, the average time spent in a company is around 3 years in the Valley, which means you will have a lot of turnover in your talent pool which has a hidden cost (training, ramp up time, etc..).