While I am not sure what the factors are for being sandboxed, but everyone has their theories. Here are mine:
Sandbox factors:
1.
SEO pages but do not over do it.
2. Dont make drastic changes in site design.
3. Get good relevant IBL's, mix it up between low/no
PR and high
PR.
4. Do not target only competitive keywords.
5. Update your content regulary but keep it theme consistent.
6. Directory listings
I believe sandbox is geared more towards link age factors with a twist. Get too many IBL's too fast and their is an extended stay. Other would disagree but eventually the site will come out regardless.