The Sandbox is a name given to an observation of the way Google ranks web pages in its index.
They key note is "observation". There have been lots of talks and debates as to whether Sandbox really exists or not. But for me, this is just a word used to name the scenario where Google (might) observes your site for the mean time depending on its quality. But several case studies shown that there are lots of loopholes on this premise.
There are cases that have been observed that not all new sites are entitled to go through this process. The thing that might have affect this is the pre-launch promotion activities by the owner. Google is smart enough to know where the direction of your content is headed and the subject of it. In short, creating buzz, or mentioning the brand or domain name can avoid your site from being sandboxed. This argument speaks that if Google is really implementing Sandbox to all sites, then it should be doing it to all regarding on the effort before the site is launched.
Less authoritative sites are being buried not because of sandbox, but because of them being low quality. The main goal of Google is to give users the best information as possible based on the data from their Data base index where our sites are located. Given the factors and the efforts, low quality sites are surely going to drop its ranking. Penguin and Panda are two of the best evidence that sandbox doesn't really exists.
Note: Those are just deep observation from different angles. I or the information where that come from might be wrong.