As mentioned due to compiler support, we can't just go ahead and use the latest and greatest new features. The moved towards non-optional C++11 usage in SFML 3 is already a slightly risky move, since it will "ban" older compilers such as VS 2010 or GCC <= 4.7.x.
But nothing has been written in stone (not even how far C++11 usage goes), thus if SFML 3 is very far in future (let's hope not!) we might look at C++14 again.
Looking at the current feature list however, there isn't really anything that would be highly interesting for SFML itself, the features are rather some highlevel additions to sone C++11 bits.