Borderlands 2 - Preview @ Eurogamer

Eurogamer has a Borderlands 2 preview based on a short demo session that focuses on the combat and what they saw:

The UI has been improved in the menus as well. No longer will split-screen require you to go through the headache of scrolling across menus and inventory screens. It's not long before we find an outpost full of bandits. Unlike its predecessor, wounded enemies in Borderlands 2 react to their injuries with knockdowns, staggers, and stuns being commonplace. There's still a slight disconnect between them being shot and their muted jerky reactions, but the correlation between your bullets and their bodies is a step forward at least.

Their AI is more advanced, too. "They can now go anywhere you can," explains Gibson. They'll hop up roofs and give chase, so you can no longer use ladders as a cheap getaway. Different enemies will have unique capabilities like healing, so they'll replenish their comrade's health if you don't eliminate them in time. This should add a greater degree of tactics to the combat than shooting everything that moves until it stops.