http://youtu.be/1uo4ZncFKH8 One of the biggest reservations I had about moving to a smaller sensor camera for video was dynamic range. Dynamic range is a measure of how wide a range a camera can pick up detail between the brightest lights to the darkest shadows. And smaller sensor cameras typically don't have as much dynamic range. This range is measure in stops or exposure values (EVs).
The thing I love about my Nikon D600 for still photos is that it can capture over 14EVs or stops of dynamic range. In practical terms, this means that I can recover highlights that were slightly over-exposed and detail in shadows that otherwise would have been pure blackness.
Here's a quick comparison between the Panasonic GH4 and Nikon D600 in terms of dynamic range when shooting video. This is not the most scientific test and I think part of the reason I'm seeing more dynamic range on the GH4 is that I have it set to a flatter capture style than the Nikon with the Flaat picture control. Nevertheless, in practical terms, I'm very happy with what the little GH4 achieves in terms of dynamic range.
For more information and more scientific testing of dynamic range, check out dxomark.com