[roller]

Here's the FAQ straight from the docs:

Why does YCM demand such a recent version of Vim? ~

During YCM's development several show-stopper bugs where encountered in Vim. Those needed to be fixed upstream (and were). A few months after those bugs were fixed, Vim trunk landed the 'pyeval()' function which improved YCM performance even more since less time was spent serializing and deserializing data between Vim and the embedded Python interpreter. A few critical bugfixes for 'pyeval()' landed in Vim 7.3.584 (and a few commits before that).