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.
Fridge Brilliance: At first the ending of the book seems like a Deus ex Machina, or even a bit of an Ass Pull. However, it makes perfect sense if you're willing to accept Death as a Magnificent Bastard. Ysabell had said earlier in the novel that Death had only brought Mort on board to marry her, and never had any need of an apprentice. Having secured Mort, Death gave him just enough responsibility to screw up royally, then moved in to fix the problem by having Mort marry Ysabell. More of a Batman Gambit, since it could have gone either way, but things things turn out suspiciously well for Death despite his apparently laid-back attitude.
In fact, once we read Soul Music it turns out this is most definitely the case: Remember that Death has a non-linear view of time. And when he saw Susan immediately before turning Mort's hourglass and saving him, he winked at her, indicating he was aware of her significance in these events. Death knew what was going to happen the whole time. In fact, it may well be that he decided to spare Mort simply because he knew he was going to, creating a Stable Time Loop.
Apparently, if Death is sharing his role with someone, it's possible for him to absorb their characteristics. This is one possible explanation for why he was so different in the first and second books — at the time he was sharing a role with Scrofula, a petty, malicious disease bringer.
Growing the Beard: Many fans consider this book to be when the Discworld series started getting really good.
Harsher in Hindsight: One throwaway line refers to a member of the Ankh-Morpork Night Watch losing his way in the Shades and calling, "Twelve o'clock and all's arrrrrgghhhh....". This was a lot funnier before Guards! Guards! humanized the Night Watch members, and the revelation in Men at Arms that Vimes gives most of his pay to the Watch's widows and orphans turns it from a gag to one of the most Fridge Morbid moments in a Death novel: chances are, the family of the watchman in question is on the list of Vimes' beneficiaries.
Magnificent Bastard: We don't get to see him much, but it's pretty well implied that the Duke of Sto Helit is one. Certainly given that when he arranged King Olerve's assassination, Olerve can only admire the elegance of his method...