There are could be multiple issues that can come up during the interviews. The most common are arrests and convictions, good moral character and any other issues related to how a candidate for US citizenship obtained a green card. It is very important to disclose all the information in the N-400 citizenship application while filing it. In the worse case scenarios, the USCIS can not only deny the Citizenship application but also revoke the underlying green card.
The most straightforward requirements for getting US citizenship is to be present in the US for at least 5 years, at least 181 days every year. There are some exceptions to that rule, such as getting a re-entry permit that does not stall the residency continuity.