On the characters:
Mitsunari: His characterization is kind of different from the SW2 incarnation, in that he's not particularly rude or disrespectful to others. He's just kind of dumb/naive about interpersonal stuff and politics. For example, he is shocked to hear that there is a possibility of Kobayakawa Hideaki betraying them, because he was Hideyoshi's successor. Yoshitsugu points out to him that, duh, it's because he WAS the successor and is now NOT ANYMORE that he would betray them. Mitsunari also makes really awful puns sometimes.

Tamaki: Sakon's daughter. While Sakon is on the battlefield, she's Mitsunari's aide and bodyguard. (Mitsunari is pretty useless at fighting)

Yoshitsugu: Just like in history, he dies fighting against Kobayakawa's treachery. I was wondering if he would die or not, but in the game it's mentioned he's terminally ill anyway.

Sakon: !!!! I was pretty shocked that Sakon dies. 0_o (This happens quite a ways in. I had thought that if Mitsunari was going to win, Sakon would also survive, but instead he gambles everything on going for Ieyasu, but because he was wounded, is unsuccessful. T_T

Kobayakawa Hideaki: Not as sweet as he looks, ahaha.

Ukita Hideie: At first, he just seems like an arrogant jerk, but then he becomes one of the more useful generals, unlike the Shimazu, who try to avoid doing anything helpful whenever possible.

Shimazu Yoshihiro and Shimazu Toyohisa: Wow, Toyohisa is no longer a generic like he is in SW2. So far the Shimazu clan don't really do too much until chapter 4. They only participate grudgingly, which is pretty irritating.

So far, most of the Eastern Army hasn't gotten that much development. Okatsu pretty much shows up and bullies Tanaka into fighting, Naomasa Ii just goes on and on about Justice!!!!!! and Fukushima rants on and on about how hates Mitsunari.

Hosokawa Tadaoki is there to avenge Gracia's death, it seems, but in this game Mitsunari wasn't responsible for it, so once you convince him of that he mostly stops bothering you.

The one who seems to be doing most of the work is Toudou Takatora, who sends ninjas to play dirty tricks on the Western side, as well as convincing people to betray the Toyotomi. He sort of has a rival thing going with Yoshitsugu, whom he respects. It would be pretty interesting if Takatora became a character in a future SW3. He served under a lot of people, and it would be nice to have more characters fighting on the Tokugawa side at Sekigahara who are not Ieyasu's retainers.

There are also some extended flashback sequences, one in which we get the whole Gracia story, and another after Maeda Toshiie's death where people try to assassinate Mitsunari and Mitsunari meets with Ieyasu. Ieyasu believes that the Toyotomi cannot rule Japan because of the lack of unity among the retainers, and that he should establish his rule for the sake of the people.

If people are interested, when I get to the end of the game I'll post more spoilers.