When I found out on Github that Symfony 2.5 improved email validation in some way I immediately wondered how this would affect the user:
Symfony 2.5 offers the new feature of strict email validation. Instead of the default behaviour (checking against a simple regex) this one uses an external (composer installable) library to perform a more strict check against all criteria defined in the original E-Mail RFC.
I like the idea of simple out-of-the-box fully RFC-compliant validation. However, I like the checkMX option even better.
Therefore my personal no. 1 new feature in Symfony 2.5 is still the new option to dump the full current configuration.