"Durham, Avignon and Prague are cities sharing the distinction of buildings in dramatic settings that jointly express military might and ecclesiastical power. [...] Of the three, the drama is greatest at Durham..." - David Souden, England's World Heritage
Well, I admittedly can't speak for Avignon, but I don't think it's just my nationality showing through when I say that my first view of Durham's castle and cathedral shortly after arriving by train knocked St. Vitus into a cocked hat. And the buildings lived up to that first view. Durham Cathedral is a stout, solid, powerful building - almost more like Bankside Power Station than a cathedral - with an impressive interior. I particularly liked the Galilee Chapel (resting place of the father of English history, Bede), with its arches decorated with zig-zags giving it a Spanish feel. We signed up for a guided tour and the knowledgeable guide took us through the history and architecture of the building, with ample reference to its OUV, but we stayed for a good while afterwards looking around the cloisters and museums and generally taking the building in.
I expected Durham Castle to be a disappointment after that - it's a less authentic building, it's only visible via a guided tour with no room for independent wandering, and I've already experienced being a student at an English university so am not stupefied with excitement by the sight of a long wooden dining table. I'm pleased to say the tour was worth it, though: the University's interior has a number of interesting Tudor nooks and crannies that our guide talked us through, and the almost-a-millennium-old Norman Chapel was a small, dark, and awe-inspiring highlight.
Outside of the two main buildings, Durham is a pleasant, compact city. There's not much to do on Palace Green (a small free-to-enter WHS visitor centre, and some library exhibitions with opening times we couldn't make). The city centre is cobbled and hilly in a charming European way; we visited just before term started so it was quiet, though I imagine once the students flood in its character changes. Framwellgate Bridge and Elvet Bridge aren't part of the WHS, unlike Prebends Bridge to the south, but they are both nicer and less out of the way. The River Wear itself is easy to trundle beside for an undemanding and calm afternoon walk. Overall, I was very happy to spend a long weekend in Durham.