One of the recurring themes of the Avatar: The Last Airbender blogs was how much Azula reminded Mark of his own sadistic, manipulative sister who he's wanted nothing to do with for years, and how this really made him want her comeuppance to be as horrible as possible. But her Villainous Breakdown in the finale was so well done that he surprised himself with an Alas, Poor Villain reaction, and even started pondering what his sister might have been going through recently until he considered trying to mend fences with her.
The first few chapters of the Harry Potter blogs. After being completely burned out from reading the entire Twilight series, he was very cynical going into this one, but is slowly won over until he's completely on board once Hagrid shows up. It's very sweet seeing him rediscover a love of reading like this.
His conclusion from Chapter 32 of "The Amber Spyglass": "'Tell them stories,' the ghost says. And I think back to how this chapter put thoughts in my mind, acts and behaviors and a history I didnít get to talk about, and as soon as I read that final line, I knew that even on my part, I was supposed to tell stories, too. Maybe itís all Iím good at, but you know what? Iím perfectly okay with that."
And for the very next chapter, he was inspired to talk about the horrible circumstances behind how he lost his faith in God. This was followed by a huge outpouring of sympathy in the comments, with several people sharing their own stories and everyone making sure to tell him how much they loved him and how good a person he is.
In his reading of Chapter 30 of Half Blood Prince, we learn that Mark got a letter from his dying father, who wasn't at all nice to him when Mark was growing up. It pretty much says "Thanks for making up with Mom. It stopped the fighting for a long time. I'm proud of you living on your own. And I'm proud to have been your father."
For two chapters of The Hobbit, he tells the story through imagining Tolkien's original bedtime story to his son Christopher. It's funny and adorable in pretty much equal measure.
Reading anything about his horribly depressing childhood makes it remarkable and quite uplifting that he's ended up as such a happy, well-adjusted adult, making money by doing something he loves.
Any time he reads something good on Youtube. Naturally, people mostly make him read terrible stuff, so it's great to see him just loving what he's reading for once.
Following The Great Spuffy Meltdown of 2012, where Mark went on at length about how most of the site's moderation is invisible and probably not appreciated, many lurkers broke their silence to tell the mods how much they did appreciate their work.
"Reading your reviews of Alanna is making me as excited as if I were writing it all over again, and I haven't felt that amped up over those books in a long time! Thank you for this gift!"
During a watching of Puella Magi Madoka Magica, Mark watches the scene where Madoka is given a Mind Rape from a Witch, which led to her saying to herself, "I'm a coward..." Mark's response is to say to her "You're not a coward!". Even though she's not real, his investment into her is so lovely.
After making a game effort to run Mark Plays as a project just as big as Reads and Watches, after a few months his schedule was so demanding that his long-dormant depression came back with a vengeance. He then wrote a post explaining that it would have to be reduced to a side project that would only be updated when he could spare the time, being very apologetic about the whole thing. The comments to that announcement quickly filled up with people assuring him they accepted the situation, and the important thing was his health.
His thanking the Tortall Universe fans for being the nicest fandom he's ever encountered in the site's history, with no one at all giving the moderators any trouble.
Similar to the above moment, in the Princess Tutu second season predictions one poster (MoElskerDeg) thanked the PT fandom for being so sweet and welcoming. The response is basically sheer fluff, topped off with this comment from Mark.
Mark: Can the Princess Tutu fandom and the Tammy Pierce fandom take over all fandoms?
His account of attending Lonestar Con 2013, after being nominated for a Hugo. He got to meet tons of fans and fellow nominees (including George RR Martin telling him afterwards that no one would ever be able to take his status as a Hugo Loser away), and was quickly accepted by everyone who'd never heard of him before.
The constant cooing over Backup every time he appears.
And saying that if he ever has a daughter, he wants their relationship to be like Veronica and Keith's.
His love for Achoo and dogs in general, best shown in Bloodhound Part 4 when he spends most of the review gushing about animals of every kind, but especially dogs.
Finally getting Ice Giants after spending five Discworld books asking about them.
Going through the (quite disturbing) first chapter of Sandry's Book, he's really happy (to the point of clapping excitedly) when he realizes Daja's black, since protagonists of color are pretty rare in fantasy.
In Part 4 of Tris' Book, he spends the first paragraph gushing about how amazing the series is, how deeply he's connecting to it, and how he looks forward to every section.