Over the top gore fest, even if most of it is not even shown. Two main characters way of the beaten track. A bunch of sick and mad characters. A lot of memorable ideas and scenes, both for their visual appeal and their disturbing undertones.

If you're into blood geysers and people getting split in two (vertically), that is for you. Also included a scissored tongue.

Kakihara is not the typical villain. He first looks like he is a generic yakuza looking for revenge, but he's actually just looking for someone that can beat him up. The scenes where he [spoiler]tortures Suzuki[/spoiler], [spoiler]cut his own tongue[/spoiler] as apology, or open wide his cut mouth are mythical.
We can see how deranged he is when Karen tries [spoiler]to beat him up[/spoiler] and he just tells her he does not feel she's into it. And you're almost sad for him when he realize how useless Ichi will be for him, despite all the hopes he put in him. So much that he actually [spoiler]dreams about being beaten up before killing himself[/spoiler]. Not sure what the final scene in the bathtub was though.

Ichi is not the typical hero or antihero. Part pervert, part child, part killer, completely insane. On one hand he is so fragile that he's more like a victim of Jijii that you think need help. On the other he will [spoiler]masturbates looking at a hooker getting beaten up[/spoiler], will save her, in the most gruesome way possible, then tell her it's ok because [spoiler]he will be the one beating her now[/spoiler]. This scene perfectly shows his insanity.

Another great story point is the complex relationship between Ichi, Kaneko, and Takeshi. Kaneko sees himself in Ichi, and helps him. Probably the only person to be nice to him, and made even more significant by [spoiler]Jijii suggesting he's his long lost brother[/spoiler]. Ichi sees himself in Kaneko's son, Takeshi, being bullied and helps him, becoming his hero. And yet Ichi is the target of Kaneko's boss. And in the end, Ichi, Takeshi's hero, ends up [spoiler]killing his father in front of him[/spoiler].

Another original character: Saburo, that acts like a dog and is able to follow a scent like one.

The biggest mystery for me is Jijii. Absolutely no idea what his goals are. Personal revenge ? Just wants to create trouble ? Does he want money ?The Yakuzas know him in the beginning, so is he just a small time crook ? Did he used to work for them ? His [spoiler]hypnosis[/spoiler] looks like a crazy power, yet no one know about this. It also probably took a long time to create [spoiler]Ichi[/spoiler] as he is today. Was it all for this plan ? Weren't there anything better to do with it ? His whole gang get caught, in pretty expectable ways, and meet gruesome ends, yet does he seem to care ? No. How did they work with him on something so dangerous ?

Same with Karen. Not sure what she wanted when she goes to see Ichi. She knows how insane and unstable he is. She knows the cause. Yet she tries to mess with him. What for ? Just playing ? Or maybe was she [spoiler]hypnotized by Jiji[/spoiler] to do that ? Because what gain could she expect, knowing very well how dangerous it could be. And yet if it's [spoiler]Jijii[/spoiler]'s doing, back to the same question, what for ? What does he have to gain for this ? Did he want [spoiler]Ichi to kill her[/spoiler] ?

The ending scene seems to suggest that [spoiler]Takeshi grew up, and killed Jijii. How did he learn what happened ? How did he get strong enough, we've seen Jijii's hidden top physique ?[/spoiler]
Not clear what happens to Ichi exactly, did he let himself [spoiler]be beaten to death by Takeshi on the roof[/spoiler] ?

It IS disturbing, at all levels, but it's also undeniably genius. Of course not all credit goes to Miike since it's a manga adaptation and most of the sick ideas probably come from there.I haven't read it and don't know how true to the manga it is, but adapting the manga format to a movie is usually really hard and here it feels like something that really absorbed the manga spirit and reshaped it into what was needed for the big screen.
Again I might be wrong and it could also be just a great movie very loosely inspired by the manga.