I was luck enough to assist in this project and I must say that a lot of great discussions took place. Unlike many other top x security lists, SANS/MITRE's methodology is fairly extensive and well documented giving you insight into how decisions were made. I do want to point out that top x lists in general are alone not sufficient enough for creating extensive application security programs. If you're looking at developing an extensive application security program at your company I would suggest reviewing mitre's extensive CWE and CAPEC projects, as well as the WASC Threat Classification. On the other hand lists such as these are powerful tools for getting the word out to the masses, and the top 25 is a superb starting place for those unfamiliar with application security. 

From the announcement

"The 2010 CWE/SANS Top 25 Most Dangerous Programming Errors is a list of the most widespread and critical programming errors that can lead to serious software vulnerabilities. They are often easy to find, and easy to exploit. They are dangerous because they will frequently allow attackers to completely take over the software, steal data, or prevent the software from working at all.

The Top 25 list is a tool for education and awareness to help programmers to prevent the kinds of vulnerabilities that plague the software industry, by identifying and avoiding all-too-common mistakes that occur before software is even shipped. Software customers can use the same list to help them to ask for more secure software. Researchers in software security can use the Top 25 to focus on a narrow but important subset of all known security weaknesses. Finally, software managers and CIOs can use the Top 25 list as a measuring stick of progress in their efforts to secure their software.

The list is the result of collaboration between the SANS Institute, MITRE, and many top software security experts in the US and Europe. It leverages experiences in the development of the SANS Top 20 attack vectors (http://www.sans.org/top20/) and MITRE's Common Weakness Enumeration (CWE) (http://cwe.mitre.org/). MITRE maintains the CWE web site, with the support of the US Department of Homeland Security's National Cyber Security Division, presenting detailed descriptions of the top 25 programming errors along with authoritative guidance for mitigating and avoiding them. The CWE site contains data on more than 800 programming errors, design errors, and architecture errors that can lead to exploitable vulnerabilities.

The 2010 Top 25 makes substantial improvements to the 2009 list, but the spirit and goals remain the same. The structure of the list has been modified to distinguish mitigations and general secure programming principles from more concrete weaknesses. This year's Top 25 entries are prioritized using inputs from over 20 different organizations, who evaluated each weakness based on prevalence and importance. The new version introduces focus profiles that allow developers and other users to select the parts of the Top 25 that are most relevant to their concerns. The new list also adds a small set of the most effective "Monster Mitigations," which help developers to reduce or eliminate entire groups of the Top 25 weaknesses, as well as many of the other 800 weaknesses that are documented by CWE. Finally, many high-level weaknesses from the 2009 list have been replaced with lower-level variants that are more actionable." - MITRE/SANS

SANS Top 25: http://cwe.mitre.org/top25/(MITRE Mirror)
SANS Top 25: http://www.sans.org/top25-programming-errors/(SANS Mirror)
SANS Top 25 FAQ: http://cwe.mitre.org/top25/faq.html