Obviously the main feature is...
that VoLTE provides you with ways to charge for VoLTE traffic independently of normal IP traffic. Prioritisation can already be done without the complexity overhead of buzzword rich theoretical constructs.
My prediction is that VoLTE will die either in favour of complete normal VoIP independently of your LTE provider (this already works surprisingly well) or there will be handsets which can use GSM and LTE at the same time. The GSM network is not likely to go away as it's seen as infrastructure, so you might as well use it for voice. We already see operators running voice calls over GSM even for UMTS handsets.
Plus unlike UMTS and GSM, LTE and GSM can coexist in the same band rather nicely. So what we may see are operators deploying it in the same band and mobile stations being able to use both at the same time.