On the gameplay side, there are two big things you need to know:
#1 - You're in service to the Gods: Rather than present the game as a prequel or a bridge between God of War 1 and 2, the developers have latched on to a specific part of Kratos' timeline to tell the story this time out -- specifically, the 10 years in which Kratos wasn't fighting against the Gods so much as fighting for them.
"A lot of what we're telling with the later stories is Kratos when he's just really realized he's a puppet of the Gods, really realizing that he's getting screwed over, and [the PSP game will] flesh out when he did have maybe a little bit of faith and when that faith started to crumble a little bit -- when he started to see some cracks in the stories they were telling him," says Chains of Olympus creative director Cory Barlog.
As has become standard for the God of War series, this means players will find all kinds of ties between the story and previous God of War tales, from subtle references to out and out reveals.
#2 - This is God of War as you remember it: Or, at least that's the goal. The primary message Sony is trying to get across for the game is that it's going to be just as epic a game on PSP as it has been on PS2. The developers aren't focused on a secondary character or a simplified concept because it's a portable game.
That means you can expect many of the God of War conventions like epic boss fights, multiple weapons and magic attacks, dramatic music, over-the-top combat (with newly added combos), story-driven gameplay, action-oriented puzzles, etc.
Many of the series' gameplay mechanics will return as well. Though we're light on specifics at this point, the developers are putting in effort to prevent Kratos from feeling like he's missing anything despite the character gaining certain abilities over the course of the first two games in the series (which come after this one in the timeline).