These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Unseen Academicals
Canon Sue: Nutt. Granted, this might be a result of his story feeling a bit shoehorned in from a different manuscript at times, with all the setbacks and failures removed from there to get the book down to an acceptable length. What's left is a character who succeeds at everything he tries in ways beyond everyone else's abilities. Scene after scene is spent telling the reader about his abilities, how impressed other characters (except the bad guys, of course) are with what he can do and things usually associated with the term 'Mary Sue'. There's an attempt to justify everything and give him self-esteem issues as a flaw, but the justification just opens up more questions and the flaw isn't having any actual impact on him. At all.
Fridge Brilliance: If the orcs seem over-designed for their racial purpose as a cannon fodder slave race, remember that their creator was most likely assisted by an Igor. Igors have a tendency of exceeding expectations, and when the marthter decided to create a race of superior soldiers, his Igor probably put more thought into what makes a good soldier than he did.
The way Her Ladyship drove Nutt so very hard to teach him to be worthy, and sent the Sisters to watch him, makes sense if you recall that she's the founder of the Uberwald Temperance Movement, and a reformed vampire herself. She knows how a vampire's b-word-thirst constantly gnaws at them, pressuring them to prey on people if they don't keep their minds occupied and have others watching them for signs of backsliding, and she mistakenly assumed that orcs' aggressive impulses might work the same way.
In a very minor example, Vetinari bids Ridcully goodbye by saying "Please don't let me trespass any further on your time." To readers familiar with his usual "Don't let me detain you", this is an impressive indication that he respects Ridcully's position as separate from, but of equal stature to, his own: he doesn't pull his usual could-be-a-threat Politeness Judo on the Archchancellor.
At the end, when Nutt addresses the crowd after the game, he stands with his arms crossed. This is how Glenda stands when she's putting up a stern, uncompromising front. Nutt copies other's mannerisms to fit in, so he learned the crossed-arms pose from her.
Rincewind plays the part of the Megapode in the Megapode-hunting ritual. Even though he's back in the UU and among friends, he still ends up being chased.
Hilarious in Hindsight: An American distributor (apparently due to a complete lack of understanding of Morris dancing) once claimed Lords and Ladies was about a football team in its synopsis, to which Pratchett's response was that football was one subject he would never write a Discworld book about...
Older Than They Think: If the book was intended as a subversion of Tolkien's conception of Orcs, in fact some of Tolkien's late writings move along remarkably parallel lines, considering that orcs might have free will when apart from their evil masters and being made from ruined men rather than elves. He died before he could finally decide on one definite origin story and cosmic fate.
Separated by a Common Language: Evans the Striped's posthumous threat to make boys without their kit PLAY IN THEIR PANTS went over the heads of American readers who didn't realize he meant their underpants.
Justified - considering the Librarian's reputation and how everyone else regards him, who would be stupid enough to try poisoning him with even a sedative?
And also foreshadowed, much earlier, when it's mentioned that unlike men, who waste a lot of time asking questions like "I seem to see a banana, but could my senses be deceiving me?", an orangutan will just eat the banana.
The Librarian's habit of taste-testing anything he happens upon isn't new; it's why he's unofficially banned from the High Energy Magic building.
The Woobie: Nutt, most of the time. At least before he tried auto-hypnosis quasi-Freudianism.