I'm not a dev and know very little about inner workings of the distro, but I do know that VL7 Light will have updated libraries and a newer kernel. VL6 Light is quite aged now. There are programs you can't compile or install because they need newer libraries, etc., that can't easily be added to VL6 without breaking lots of things.
I agree that VL6 Light is a lovely thing and I use it every day on my laptop. Nevertheless, I'm really looking forward to VL7 Light. The goal with Light is to make it lean and mean while providing a complete Linux installation that provides for the things most people do routinely. I'm sure we'll see the same thing with VL7 Light.
By the way, when testing starts for VL7 Light, it would really help if you could participate. Testing as many hardware and software configurations as possible really helps to uncover bugs. If you can set up a 10-gig or so partition for future testing, you'll find that testing VL7 Light alpha/beta/RC will leave the rest of your computer alone.