MongoDB 2.4 brings a significant change to its JavaScript engine, switching from Spidermonkey 1.7 to V8, an open-source high-performance JavaScript engine from Google. This change enables improved concurrency, allowing multiple JavaScript jobs to run on separate cores, potentially reducing query execution times by a factor of the number of available cores. The new implementation also supports modernized JavaScript features like ES5, making it easier to develop and maintain migration scripts and other applications. However, this change impacts certain features, including MapReduce and $where operations, which may require refactoring of code that relies on global shell functions or properties no longer available in MongoDB 2.4.