Apple's iPad comparison page shows the spec details between the iPad mini, iPad 2 and iPad with Retina Display. Of interest, the iPad mini supports Siri while the iPad 2 does not, despite sharing the same Dual-Core A5 processor as the iPad 2.
Before the launch, there was some speculation that there may have been technical reasons why the iPad 2 was not capable of supporting Siri. The new iPad mini does have internal improvements over the iPad 2 but does share the same A5 processor. It may also be possible there could have be microphone improvements with the new device, but Apple hasn't specified.