The main difference (apart from the added "can't be regenerated) is that if a creature has deathtouch, 1 damage is always considered lethal damage regardless of the blocking creature's toughness. If for example, a 2/2 with deathtouch is blocked by two 1/3s, it can assign 1 damage to the first and 1 to the other, while if it had Komachi's ability, it had to assign 2 damage to the first blocker so the other one survives.
It matters more if Komachi has trample. As her ability is very similar to Deathtouch.
Komachi's ability will still kill all creatures she deals combat damage to, and you can divide the damage however you want. Combat damage is always divided as you choose, unless your creature has trample and you are trying to assign it to a player.
It hasn't been like this for a while. With the new combat damage rules, the attacker chooses priority, let's say Komachi attacks and Akyuu and Reimu blocks. The controller of Komachi chooses Akyuu, then Reimu in that order. Then, damage is dealt, until lethal damage is given, then it will automatically go to the next creature. Note: damage is dealt automatically, there is no damage assignment.
I still feel you SHOULD have full control, but I guess thats an argument for Wizards, not this thread lol.
For the most part, it should make no difference to a regular player. The difference being really choosing priority of the creatures blocking. It kind of nerfs sorceries that you can do after battle. Like if Attacker A(3/3) attacks, and is triple blocked by creatured B(1/3), C(1/3), and D(1/3). Attacker A can deal 1 damage to B, C, and D. Then, after combat, play Pyroclasm to kill all of them. New rules won't allow this.
Clearly the solution is to bring back banding
I don't even know how banding works with these new rules. I'm sure its mentioned in the rules book or something.