But what is it that makes the hills difficult? Is it more so pitch, or distance?
Depends on the hill. Ravine, Suffolk, B/H/E are long, steep hills. Others, like Berryhill, Canton, and Welsh, are just short and steep. A few hills have dodgy surfaces (Canton being the best known), which adds to the difficulty, especially in a group.
As Eric mentioned, depending on where you live, you can probably access a few of the hills without venturing very far (the South Side Hills are very easy to access, and if you live in the East End, there are four hills easily accessible on the other side of the Allegheny). Those hills are a reasonable cross-section of the entire ride.
Most everyone will have hills that are "easy" for them, and others that are really hard. I've never done the DD proper, but I've climbed all the hills in a single ride, and my experience is that there are certain "mental" hills for me:
* Centre: Yeah, it's not too steep, but you just look up, and you see the entire hill in front of you.
* Logan: Like Centre, but the climbing is even harder.
* B/H/E: Making the left on Eleanor and looking up is just soul-crushing.