Immediately, I could see what caused such popularity. Take a realistic, modern setting, add one solitary, yet very potent, fantasy artifact, some strong leads, a dark and tense atmosphere and a unique protagonist/antagonist relationship and you have a series that should interest even the most stubborn and particular anime fan. And even those living under rocks or who staunchly defy modern anime or even anime in general as a medium likely know, to some peripheral degree, about the series and its basic premise -- which alone is enough to, eventually, draw in any viewer as it finally did myself.
Following a brilliant young man who simply happens upon this artifact of complete destruction, the series sets itself apart by having a protagonist that, though with good intentions, is near-immediately painted as a bit of a twisted psychopath who would commit to the most dubious means in order to reach his apparently well-justified ends.... while the antagonist, a world-class detective tasked with catching this young man (and ironically about the same age himself and just as thoroughly gifted), though cold and quite far from 'pure', has a more humane justice in mind, one that, in all but the most dire of circumstances, would avoid the death of any human being, no matter how sinister -- and one that clashes rather directly with the protagonist's views. Basically, the main character's the bad guy and the antagonist's the good guy, though it's a bit more complex than that. Kinda like Code Geass.. and in fact this series shares another major point with that one as well which I'll mention later... although it should be more like Code Geass is kinda like Death Note, though I personally watched the former first.
Now, I'm going to do my best to avoid any type of spoilers here, but I'll forewarn you that this and the next paragraph will contain spoilers as to the structure of the show, at least. Basically, from the very beginning to about 2/3rds through, the story is interesting, engaging, and constantly builds up, introducing and expanding upon various interesting sub-characters and weaving deftly through entertaining and gripping sub-plots, all the while keeping the main focus on the brilliant and engrossing battle of wits between the "hero" and "villain"... as well as the thoroughly amusing relationship between the main character and his death god... "friend", the original owner of the note whose job it is to follow him as long as he owns it. (Ironically, this character is the closest thing to a mascot or comic relief, despite being the only physical 'monster' in the initial cast...) The anime continues this trend, each mini-storyline becoming more interesting and tense than the last, threading the overarching plot forward bit by bit without pushing things too far in any direction; the raw genius of both leads and the prowess with which they're written equates to a story that manages to feel as if it's satisfyingly moving forward, while no real weight is put into either corner, allowing things to become further and further exciting and riveting with each passing episode, but without interfering with the delicate balance of the wondrous cat-and-mouse game.
But then, most suddenly and for seemingly no good reason, something happens. About two-thirds through the series, an event abruptly occurs which thoughtlessly renders several of the most interesting and enjoyable characters in the entire series moot for the rest of it. Chief among them being a particular character who I did not expect to like very much at all, but who ended up growing on me far, far more than I would possibly have expected. That being said, after this point many new and very intriguing characters are indeed introduced, but given the honestly sloppy and rushed feel of this last leg of the journey, they mostly fail to live up to the incredible potential that they might have had. Plus, one of them is just too shameless of a lesser clone of one of the extremely likable and memorable previous characters, so much so that even trying as I might, I couldn't force myself to like him very much at all. Quite the opposite effect of the predecessor which I mentioned having a turnaround about a bit ago, in fact... And yes, as mentioned before, this is the other point this series shares closely with Code Geass. In many was, the last third of it feels like R2. Either way, the finale of the series ended up being quite satisfying in some ways, and did help to redeem these characters at least somewhat, but for all it did well it feels like it fell short an equal number of ways...
When all is said and done, Death Note is a thoroughly fascinating series with enough thrilling moments to warrant a watch, if only to see what all the fuss is about and be able to acknowledge that you, like the apparent majority of anime fans by this point, have watched it. And for the majority of the series, it really, truly is crafted quite masterfully, to the point at which it even pains me to give it less-than-stellar scores... but I cannot overlook the fact that the latter story arc came so abruptly, came about so disappointingly and was handled so poorly, that my infatuation with the series very quickly dwindled down, like the flame of a candle being blown out to leave only a cinder on the tip of the wick. To put it in one blunt but factual way, i went at that point from watching about five episodes a day to watching about two episodes a week. Not good.
Even still, I can't go without mentioning that the artwork for the series is quite nice; many characters have unique and memorable designs, the use of subtle facial expressions to great effect as well as indulgence in completely over-the-top expressions in other scenes done equally well is great, and the rarely-used but done-so-with-great-effect "action shots" are a treat. As well, the soundtrack is quite superb, from haunting melodies to glorious choir pieces to understated "detectivework" themes and catchy character songs; "L's Theme B" is a particular favorite.
The long and short of it is that Death Note is a good series and is worth a look if you bear even the slightest interest in animated thriller; though the last act mars the overall series, its quality is high enough that I can say with certainty that it's worth a look.. and for its shift in tone to bother me so much, that only further proves just how wonderful the majority of it is.
