Bluetooth for a start. It's probably the result of me driving a lot, and am quite pedantic about trying to answer my phone every time (work related), but I just find having hands-free a must in my car. Even more-so when you are already swapping out the head unit, why wouldn't you just go for the one with Bluetooth built in? I don't generally stream music from my phone, as I usually rock USB stick for that, but having the capability to do that is nice too. Most stereos today will have USB and Aux in. The need for an actual CD player then becomes mostly redundant, since it is usually much easier to just hook your phone up via either BT, USB or Aux in, or use USB sticks, or Ipods or whatever than carrying a pile of CD's around.
Other than that, I just prefer the sound I have had, in the past admittedly, of Kenwood or Alpine car stereo than I have had from Pioneer. I'm sure things have changed, but it's just my opinion. I'm sure JVC make cracking head units too.
As for DAB, that's digital radio. I use it a reasonable amount in my 5 series, so I am leaning towards going for a DAB unit in my SEAT again. But then I'm not sure I really want to spend an extra £70 for it, lol.
I'm by no means an expert on car audio though. And I couldn't tell you the differences between a £100 stereo and a £300 with the same feature set (on the surface at least). But I generally just look for the Kenwood or Alpine with the features I want, and decide between them.