Perhaps the best way to start is by saying that there aren't many (good) animes out there which dare to tell its story in a different way from the usual, in which there isn't a proper protagonist. Baccano! does this and shines on that aspect.
* Animation:
Baccano! does well in terms of animation quality at most of the times, mainly during action and dramatic scenes. The color pattern used is well chosen because it enhances the idea of its story happening in the 1930s, you can easily realize the use of washed out colors and the predominance of brown in the pallete.
* Sound:
The OST is well chosen for the series, it helps to transmit the 1930s ambientation. The OP song is a unique instrumental jazz song and it's incredibly good and fits the series perfectly.
* Characters:
This is surely the department where Baccano! masters completely. This show has the best cast of likable characters I've ever seen to date. On the first episode I was thinking to myself "Whoa, there are lots of characters here, can they manage do develop them in a good pace through 13 episodes?". Yes, they could. The characters go from the extremely funny couple Isaac and Miria to the psychopath Ladd Russo. There are also lots of interesting couples in this show.
* Story:
This is where it gets "tricky", Baccano! doesn't follow a chronological line of events. In fact, it time lapses many times within the same episode between 1930, 1931 and 1932 events. At the beginning it's a little frustrating how you cannot understand many things if you're not paying a lot of attention, but as the anime progresses, things start to become clearer. If you like the 1930s stuff envolving mafia families, heists and conflicts, this anime is for you :)
* Overall:
I couldn't give Baccano! a different rate other than a 10. It has everything I could ask for: Great story, awesome ambientation, pretty good OST and the best cast of characters I have yet seen.
