The consular officer looks for the following in a visa applicant:
1) That there is a genuine reason for this person to travel to US,
2) That there is a overwhelming reason for that person to return,
3) That the person has adequate financial resources to undertake the trip,
4) If the visit is for family reasons, the family member in the US has proper status in the US and (if necessary)has the financial resources to support the visitor.
Item 1 is obvious and in the visa application/covering letter the exact reason to visit must be clearly spelt. The leter should also include any travel plans within US (visiting places, etc.)
Item 2 requires the visitor to show that he/she has strong ties to her country i.e. has close family members, financial assetts, job, etc. that requires him/her to return. Documents include employment/income information, property ownership info, bank accounts, etc.
Item 3 is partially covered in item 2 above. If the visitor is depending on financial support from US based family member, then documents supporting employment/business and other financial assets of the US based sponsor is required. Pay stubs, bank statements, employment letter all help towards this requirement. Most US based sponsors also supply notarized form I-134, affidavit of support to support the visitor's visa application.
Item 4 requires immigration status documents of the sponsor like green card, H1, copy of pending 485 application, copies of ALL pages of sponsor's passport, etc.
In spite of all this, sometimes the consulate will reject the visa for no apparent reason.