(There is a spoiler behind the cut-tag. If you don't want spoilers click here and scroll past the entry about Maastricht.)
The last Pratchett I read which I felt had real relevance to my work was The Fifth Elephant, and so I shouldn't be surprised that Thud!, which revisits some of the same themes and characters, rang particular bells for me. Indeed, outside the works of ianmcdonald and one solitary novel by Mildred Downey Broxon, I can't think of any sf novel that comes as close to tackling the Northern Ireland question as does Thud!. (I've always been convinced, mind you, that we are meant to understand a strong Northern Ireland subtext to Life of Brian. I may enlarge upon this at some point.)
There is the obvious point about the commemoration of a bygone battle, remembering 1690, 1916, or whatever. There is the obsession with visual representation of the battle. We are told that the descendants of the two sides like to engage in commemorative parades and also commemorative punch-ups. It's all rather familiar.
Not that I'm saying we are meant to read Thud! as anything other than a commentary on intolerance and bigotry in general, rather than on one particular historical or geographical instance of it. To take one possible alternative reading, I'm well aware of the significance of the 1389 Battle of Kosovo (having recently revisited its site), which also has a famous visual representation attached to it. To take another point, the theological debates of the "deep-down dwarfs" are clearly meant to be more reminiscent of debates in contemporary Islam than of anything else. (Though such repugnant fundamentalism is not restricted to Islam.)
There are points of departure, of course. In our world, people tend to read their history at different rates. So, for Ulster Loyalists, 1690 is the big date; for Irish Nationalists it's an irrelevance, a struggle between two foreign kings, and the big dates are either from the twelfth century or (more often) the twentieth. (Though even there with certain omissions.) Likewise, for Albanians, the big anniversary is the end of November not July, commemorating Skanderbeg and the first raising of the Albanian flag. And if both sides actually do agree on the crucial date (Bosnia 1992-95, Cyprus 1974, Israel/Palestine 1948) it doesn't really mean that solving the problem gets any easier...
And of course, most important, this is a Terry Pratchett novel; so we pretty much know from the beginning that it will all turn out to have been a horrible mistake, with the winning factions on each side turning out to be those that are in fact dedicated to peaceful coexistence. If only real life was as easy as that.
There were other things I liked. The idea of the wargame, Thud!, turning out to be something that drew people together seemed instinctively right to me. My main such activity growing up in Belfast was the School of Music; I myself ascended to the dizzy heights of second percussionist of the City of Belfast Youth Orchestra. But I know that the Modeller's Nook on Winetavern Street was a focus for wargaming across the barricades. At about the same time there was a brief boom in Northern Ireland-based postal Diplomacy fanzines, of which the best was probably Philip Murphy's Morrigan, which alone had a determinedly (perhaps even unconsciously) cross-community ambience. I'm sure Northern Ireland's chess club federation is similarly non-denominational. (Unlike, interestingly, the Scouts.)
I also liked the description of the battle panorama as a conceptual breakthrough devised by an insane architect. I can now comfortably predict that for the next few decades, casual visitors to the venerable panorama at Waterloo (or the less well-known one in Lucerne) will turn to each other and say, "Gosh, I wonder if they got the idea of doing this from Terry Pratchett?" (Is there any such panorama in the UK - especially of a military nature?) Interestingly, the deranged artist expiring in his studio also has a Belgian precedent (though unconnected with the Battle of Waterloo as far as I know).
And of course particularly gratifying was the spoof of the Da Vinci Code, a book with no virtues and much fodder for conspiracy theorists. Except that of course Pratchett's version does turn out to have some validity in the end. Hmmm...