Most of the pre-meds that I know who did not get in changed their career prior to graduation from undergrad. The biggest thing I saw that hindered people from continuing along the pre-med track was poor grades. I knew someone who failed a class so many times that the department told this individual that they had to switch majors.
The MCAT is another big hurdle. The people I know who did poorly on the MCAT took it before they were ready to take it (i.e. their practice tests were low). However, they have since chosen other careers and are happy.
Another common mistake is that people are not strategic about the application process (i.e. applying to one school only, applying to state schools where they are OOS, applying late, etc. etc.).
Trying hard is not enough. You have to get the grades, MCAT, and then make good decisions when it comes to the application process. A good advisor can be very helpful; however, many colleges do not have good pre-med advisors. My pre-med advisor at my college was not helpful at all which is why I turned to SDN.