In The Long Halloween we get to see the downfall of Harvey Dent and how he ultimately becomes Two-Face. I was never very interested in Dent or Two-Face, to be honest, and the Batman Forever film might be to blame.
However, in this comic I found Dent to be a fascinating character. You can see how the perversion of the city gets to him little by little. It's difficult for me not to compare the stories with Year One because it was the first one I've ever read but, in my opinion, Harvey's feelings, seeing how his city is getting more rotten by the day, are very similar to how Gordon felt as he moved to Gotham in Year One. That despair and the sense that nothing can be done to change the ways of Gotham are what finally breaks Dent.
I also liked the side characters in the story. Barbara and Gilda help us get a glimpse of the effect all the killings had on Gordon and Dent. Alfred continues to be awesome and I keep wondering why the title of the world's greatest detective is not given to him, instead of Bruce.
The mystery part of the story didn't appeal to me as much as seeing how Dent's character develops throughout the comic. As it happened before, I felt the investigations are an excuse to see the characters' reactions and interactions and did not interest me as much. Even though I wasn't as interested in the mystery aspect, I was still trying to figure out who the culprit was and I love the plot twists at the end.