1: Shingen, Traffic Cop, and Kasuka were all pretty much pointless characters - which would have been O.K. were they not featured in the intro. Seriously, Kasuka doesn't even make an appearance in the second part, yet he's in the intro. I don't get it.
2: No resolution on the Dullahan = Valkyrie plotline whatsoever. It's not even referenced again after it's established. And that was the basis for sparking the whole gang war. The purpose of the events of the entire series doesn't get a resolution. Even if they did somehow thwart Izaya's plot with no knowledge that it even existed - he just gives up? Someone as clever and resourceful, not to mention maniacal as him just gives up?
3: In the final fight, all the Dollars members hang around doing nothing until Kida is half dead before they bust out hero mode - and over half the group there were thralls to Saika, but she doesn't care enough to give them an order? Not to mention the super-convenience given to Celty dragging them together and telling them how to solve their problems. Anyone with basic story structure know-how knows that's poor writing.
4: Kida - OUT! I really don't get this ending. Kida had finally come of age as a character and faced himself and his guilt, which were brought about by his "Running away" when his lady needed him - but then in the end he leaves without saying goodbye to his friends. There's only one reason he wouldn't be able to say anything to them, and then be staking their chatroom trying to be secretive - and that would be him feeling guilty. So... after just having his epiphany, he remakes the same mistake? So as a character, he did zero development?