Great themes. Many memorable lines. Interwoven stories of many fascinating characters. Extremely interesting relationships.
And the second most awesome antagonist I've ever known, next to Joshua Kiryuu (with whom I swear they would be BFF).
What's best about Izaya is that I've never seen another antagonist like him. Oscar Wilde's line, his favourite quote, really describes his nature, I think: "It is absurd to divide people into good and bad. People are either charming or tedious." What's amusing is that he is both endearing, drop dead sexy, and very annoying (even that is endearing). He's obnoxiously philosophical, never shuts up, profound, and very, very human.
What drives him isn't some cliched hatred or revenge, but a twisted love for humanity. He doesn't have a backstory--he grew up in a normal family.
Let's not forget the most badass ex-bartender of all time, Shizuo. There are no words to describe how epic he is. Rather, he doesn't use words, since he is violence incarnate, after all. I'm not sure what he is, because he's not a traditional tsundere. His character is also very interesting, but my complaint is that in more recent volumes,
he's changed. It's for the better (is it really?), but it makes him a very dull character. His romance with Vorona, from what I know, is predictable, cliche, and boring.
He should end up with either
a) someone who can keep him calm, who never riles him up (Kasuka, Tom, Vorona.) Someone that isn't afraid of him, who he isn't afraid that he'll hurt (which is impossible, because he can't control his strength--this way would change who Shizuo is).
b) someone who drives him completely over the edge (you know where I'm going with this). Someone who he isn't afraid of hurting, someone who matches his strength with another kind of strength.
Mikado, aka Bosskado, makes for a wonderful decoy protagonists (I didn't even know those existed!) The trio just gets more and more interesting.
Ahh, Shinra and Celty are one of the few NL pairings I actually support. For good reason, too. You'll find no other couple like them. They're very amusing together. Generally, the fandom's favourite quote is from Celty:
"Because the world isn't as bad as you think."
This series has its flaws, of course, but overall the most important factor is there: It's super interesting.
I consider this one of those series that people can't go without reading/watching.