Okay, so let's take a step back. What does mirrorless give you that a DSLR doesn't? Higher frame rate and the ability to use the viewfinder when shooting in situations where, with a DSLR, you'd need to lock the mirror up, like shooting video, correct?? Oh and eliminates mirror slap for long exposure stability.
It makes the camera slimmer which is nice if you like lightweight, compact systems, particularly when you're using smaller lenses.
You can preview your exposure/exposure compensation, chosen film simulation, DoF, histogram, MF Focus Peaking etc etc all live before and during taking a shot. I can't emphasise this one enough. I have used the exp/comp dial more nice getting a mirrorless system than in the previous 40 years of using a camera, it is now so intuitive I tweak the histogram on more shots than I don't.
You're not to limited to one brand of lens fitting - you can buy £15 adapters to fit just about anything you can possibly imagine on the front and still retain both infinity focus and full metering.
On sensor AF isn't limited to a narrow area like most phase detect systems so you can place your chosen point anywhere in the frame - like live view on a DSLR but without the handicaps that come with it.
Apparently with this new Sony - NO - viewfinder blackout which if that is the case is a major leap forward....if we carefully step around viewfinder systems and TLR's of course