One, in hindsight they are full of great big anvils about what is going to happen to Donna. The look on River Song's face when she realises who Donna is; the ending of Silence in the Library with Donna's face on the node saying "Donna Noble has left the Library; Donna Noble has been saved" when, in order to save Donna, the Doctor has to make her 'leave the Library' and basically wipe her mind of all the information she's acquired over Series 4; the fact that Donna spends most of Forest of the Dead living in a fantasy world because Doctor Moon has put her brain to sleep and how she will live the rest of her life with a part of her brain sleeping; Lux's statement that "a half life is better than no life at all" which must influence the Doctor where Donna is concerned in Journey's End. Oh yes, there are indeed rampant spoilers in these episodes, but you only see them when you rewatch.
Two, there is a massive love of reading and books running through the two episodes which every bookworm will love. CAL is a descendent of both Alice and Dorothy Gale, exchanging a real world for a fantasy one. The scene were the Node delivers the Head Librarian's message to the Doctor and Donna is an echo of the scene in Fellowship of the Ring where Gandalf reads aloud the last days of the Moria dwarfs, ending with the ominous "They are coming." And they (the shadows) are coming. Doctor Moon telling CAL "the real world is fantasy and your nightmares are real" - come on, this is what every child suspects! Books can be more real to children than life because they contain superreality and truth and you never forget the books which mark your life and mind, whether they're well written or badly written.
Three, these two episodes ARE like a 'Greatest Hits' set for Moffat, but since they were written about the time he knew he was going to be taking over as showrunner and David was deciding whether to stay or not, I think they are like a showcase for him. Here is what Moffat can do: scary monsters; meta writing that isn't clumsy (the whole discussion on spoilers between the Doctor and Donna, then the Doctor and River Song); strong (if irritating) female characters; strong foreshadowing; multiple layers; bittersweet endings; moments of true horror - the moment where Donna's children disappear from their beds in the blink of an eye has to be one of the most terrifying moments in New Who, I went cold and I don't have any children; messages about the thoughtlessness of humans (the forest of the Vashta Nerada must have covered a whole planet in order to produce that many pages, and yet there was no research into the ecosystem and lifeforms of that particular planet, which also ties in nicely with the theme of planets disappearing) balanced with how humans can do great and wonderful things (River Song sacrificing herself).
Four, the Doctor is put in the position that Reinette occupied in Girl in the Fireplace: this mysterious person appears in his life, knows everything, won't share that knowledge. I think River Song was deliberately designed to be a little irritating: that's how the Doctor often comes across, jumping in and rattling off orders, telling everyone he knows best. It's a bit different when someone does that to him. And even though you get the repetition of "Everybody lives", it comes at a price. Unlike in The Doctor Dances, this time, people are not revived and reborn. Their bodies remain dead. The Doctor can only give them a half life, but it's still better than no life at all. Just as he will do for Donna.