Mark Rein (he mentions Unreal Engine 3 within 20 seconds, incidentally) says in the audio track that it only took him about an hour to figure out the arrow system, and I can see what he means - arrows twist and change shape to represent changes in height and direction relative to your position and orientation. There's no mini-map, but there is a bottom-right compass, which will show up markers for other players but only when they do something - like sprinting or firing a weapon. If they hadn't mentioned the fact there's going to be a regular big map, I wouldn't have thought to ask about it - the game doesn't call out for one at all.
Speaking to me afterwards, Chris Collins admits the game won't have the verticality of Crackdown, Realtime's Xbox 360 side project of a few years back, but says it matches and betters other contemporary open-world standards. There's evident variety in vehicle handling, armour levels and speeds, adding a bit of dynamism to car chases when battles go mobile, and combat situations are full of players crouching behind cover and clambering over the surrounding level furniture for a superior vantage point, all while a handsome day-and-night cycle (about eight real hours for a full one) bathes the reasonable draw distance of San Paro's waterfront district in a procession of dusky hues.
But it's the fact of the other players that distinguishes APB. They aren't just working through the content like you are; they are the content. They're responsible for the enormous variety of outfits and vehicles, their actions have unlocked the diversified weaponry that characterises your mission-based combat, and thanks to some cunning VOIP software, their real voices will fill the air between gunshots if you're within a logical radius. Even their taste in music determines the game's atmosphere: thanks to last.fm technology, a gangbanger cruising by in his SUV fills the air with hip hop; he may be listening to Dr. Dre, but you might hear NWA if last.fm's matchmaking tech decides that's the closest equivalent on your hard disk. When you die, you hear death music they wrote in the music creator.
For all the interaction, on servers running the standard ruleset you can't just mess with players for no reason, even if they're on the other side. People with grey names on screen may well be fighting one another as you drive past, but you can't run them over, or get out of your car and shoot them - if you do, your car or bullets will pass right through and they won't be griefed. Your direct involvement is a product of matchmaking and, you know, the law. If an Enforcer spots a grey-named Criminal breaking into a car, Mark Rein can shoot them, or slap on the cuffs for a greater reward, the game having alerted Mark Rein to their nefarious activities.
The matchmaking looks smart, and uses the game's up-and-down experience system for matching people of suitable skills. As you play you can ascend to level 15 or fall back down to level 1 depending on your fortunes. If you and a fellow high-level Enforcer are tasked with escorting a package, the game may send four nearby Criminals of mid-level standing to block your progress, for instance.