USCIS publishes statistics on the approval of deferred action applications, but only periodically; the last publication appears to be November 15, 2012. The statistics are available here: 2012-1116 DACA Monthly Report, for review, but it is important to note the following:
Applications are rejected overall about 3.2% of the time. Applications can be rejected for any number of reasons, including something as simple as a missing signature or attachment. The forms used to apply for deferred action are quite specific about documentation needed. Rejections are also not the same as denials, which are categorical denials based on the rules published by USCIS on eligibility.
Acceptance into the program is running only a 17.2% rate at this point; most applications are sent for processing, biometrics, and background checks. There was an explanation in a recent meeting with USCIS that the service may request additional information, called a Request for Evidence. Some applicants may be asked to update or supplement information provided. The USCIS is hoping to process most applicants within four to six months from the date of filing; meaning most will not be approved until potentially February, if they began in August when the program began.
Information and updates from the USCIS is available in Stakeholder meetings, and a link to notes from such a recent meeting on November 19, 2012 is available here: Lawlogic Blog. The USCIS again expresses that most traffic offenses do not qualify as misdemeanors, even if they are so categorized under state law. They indicate there are no statistics available on denials, those applications which are adjudicated by the service as failing to merit a grant of deferred action; however, the service now indicates these applications can be reconsidered if new or amended information is available for any applicant.