The theme playing at the very beginning of the pilot is a chanted variation of the taize prayer Kyrie Eleison, which is Greek for "Lord have mercy."
The theme that plays at various key moments - the "I am Justice" duet, the capture of the Yotsuba Kira, the revelation of Light's final gambit against Near, and the timeskip montage (where it plays in full) - is a Dies Irae. Oddly, it does not include the fifth verse of said apocalyptic choral number ("The written book will be brought/In which all is contained/Whereby the world will be judged"), but that is covered in "Teleology of Death", which plays when less overblown Ominous Latin Chanting is called for.
In the first episode (blink and you miss it) one of the names written in the Death Note is Uwe Bnote oll?...
One subtle indication of Light and L's opposition: Light uses a PC whereas L uses an Apple Macintosh.
In episode 24, when L pilots a helicopter, he sits normally. There's an in-universe reason for this, as helicopter controls cannot be used from L's usual sitting position.
During the English dub scene in the helicopter in episode 24, when L is holding the Death Note and giving a stream of thoughts showing how this notebook proves that Light is Kira, that last thing he says at the end of the stream is 'Yellow Box'. As this is the name of the abandoned warehouse mentioned and used in episodes 36 and 37, this can either be considered an accidental spoiler or epic Foreshadowing. In original audio he just says "two notebooks".
The "13th" issue of the manga (basically a Death Note encyclopedia) notes that any time L gave a percentage of the likelihood that Light was Kira, he was lying— it was always 100% - in fact, "any time L throws out a percentage, it basically means that he suspects that person by over 90 percent." (this still doesn't explain why L throws out the same statistics even in his inner monologues) It also notes that he was lying when he called Light his friend.
L has enough traits to be diagnosed with Asperger Syndrome. Same with Near, but he doesn't have enough traits to be albino.
Even though he is shown mostly sitting, Near is shown standing while meeting with the President.
Minus Takada if you count her as the fifth Kira, the Kiras die in alphabetical order. Higuchi is killed by Light, Light is killed by Ryuk, Mikami dies by Near's actions (I follow the Near manipulated Mikami with the DN theory) and a year after Light's and Mikami's deaths, Misa commits suicide. H,..L, Mik, Mis... Clever, eh?
It's the same with L and his successors. L, then Mello, and eventually Near. So it's L, M, N.
Mello and Near's character designs were accidentally switched during character creation explaining why they have Ironic Codenames-they were originally going to be Meaningful Names-Mello was intended to be the "mello" one and Near was intended to be the Axe-Crazy one who was drawing "near."
Aruno Tahara, who voices the US president in the Japanese version, previously dubbed George W. Bush in Fahrenheit 9/11.
In the Mexican Spanish dub, Misa Amane's voice actress (Rebeca Gomez) voiced Mandy, a girl who's friends with the western version of a Shinigami.
Ascended Fanon: Ohba originally had no intention of meaning anything when L kicks Light in retaliation for being punched, but when fans identified L's kick as a move in Capoeira, Ohba decided to just Throw It In! and take over the idea that L was a practitioner of said martial art.
Banned in China: They're 'a poison which creates wicked hearts', apparently. Mainly because it inspired students to alter notebooks to resemble Death Notes and write the names of the teachers they hated in the books.
Creator's Favorite: In the Mexican Spanish dub, L is one of the favorite characters his voice actor, Hugo Nuñez, has voiced, to the point he even tattoed the "L" letter used in the series on his body.
Defictionalization: Yes, there are real life Death Notes available for sale. No, you can't murder people by writing their names in them (hopefully...)
Fandom Nod: The interstitial factoids in the manga.
Flip-Flop of God: In the How To Read volume of Death Note, Tsugumi Ohba (the writer) stated the L was the most intelligent character in the Death Note series because the plot required it. However, in the very same book, his intelligence was rated 8/10, compared to Light and Near's 9/10.
No Export for You: The one-shot chapter has never been released in North America. Also unreleased in are the three DS games produced by Konami, four of the DVD collector's figures (Soichiro, Takada, Mikami and the King of Shinigami) and the anime guidebook. Luckily, Viz released the one-shot with the Omega Edition Blu-Ray set of the show.
The Other Darrin: The TV drama uses a completely new cast, recasting even Ryuk who is voiced by Jun Fukushima instead of his original voice actor, Shido Nakamura. This, along with the musical, is the only time that Nakamura has not reprised his role.
In the series English dub, the only inconsistency was the FBI Director, Steve Mason. For his phone call to L, Michael Dobson provided his voice, but after the Time Skip it was changed to John Novak.
The author originally intended to use Mogi for an awesome and epic plot development, but could never figure out what that would be (beyond it being awesome and epic).
According to How To Read 13, the entire task force were going to die in heroic ways. In the actual story Matsuda, Aizawa, Mogi and Ide all survive.
Ryuk was originally going to look like this: ◊ but the creators were worried that if he looked cooler or more attractive than Light then the fans wouldn't pay attention to Light.
According to How To Read 13 for that scene where Light and L are "getting to know each other better" the creators were originally thinking of having Light and L do a fencing duel before ultimately settling for tennis instead.
Also according to the How to Read 13, Mello was supposed to be the one who defeated Light, not Near. Ohba ended up killing him off though due to him learning too much about the Death Note, meaning if he were kept alive much of the tension in the series would have gone kablooie. Both Ohba and Obata admitted to liking Mello better than Near, with Ohba saying that being told Obata also preferred Mello probably would have been enough to change the ending back to Mello winning.
Naomi Misora was also originally going to be more of a major character, but Ohba had to kill her off quickly as it was felt she'd complicate the story by being too smart and figuring things out fast.
How To Read also mentions that in the planning stages it was briefly considered that Mello and Near would be L's twin sons but that idea was scrapped, likely as it would mean someone like L actually managed to have sex with a girl and the author just couldn't see that ever happeningnote as well as with L being 25 at the time of his death and Mello and Near being young teenagers when they're introduced, that would have brought up some Unfortunate Implications about the age of their "father", so Mello and Near became Tyke-Bomb orphans instead. Mello was also going to have white hair, as seen in an early color poster featuring the younger versions of him and Near (on the main page), but had his hair changed to blond by the time Obata started doing color images with his adult self.
The high-ranking Daril Ghiroza was originally going to play the role that Sidoh ultimately did, but this was scrapped because of her complex design, and her having a high ranking wouldn't look good against her being "pushed around" by humans.
Tsugumi Ohba's and Takeshi Obata's next series, Bakuman。 had a thinly veiled Self-Deprecation in which a series that was a stand-in for Death Note, was be pressured by the Editor to continue dragging the story out past the main rivalry. This leaves the impressions that Ohba and Obata originally did not plan for the series to continue pass the "Light vs L" plot.