PC users playing side-by-side with Xbox and PS3 users in a shared, persistent massive MMO universe - advantage, mouse and keyboard users. I was fairly flabbergasted that Trion Worlds' San Diego-based offices already have the three platforms playing together so nicely. In the live demo shown to me Tuesday morning at E3, all three platforms were represented (the Xbox and PS3 were shown side-by-side, while the PC version was played by developers remotely from their offices in the game).
The art style and gear design reminds me of a cross between Halo, Crysis, and a dash of Dead Space. The setting is a lush, semi-post-apocolyptic earth, where the aliens have come to terraform the Earth, instead of incinerate it - which makes this the most green post-apocoltypic landscape I've ever seen. The environment design may best be described as Borderlands with plants.
And like Borderlands, you're playing in a massive open world shared by other players that you can explore and discover quests and remnants of civilization to interact with. Unlike Borderlands, you're not sharing this game world with a half-dozen other players, you're sharing it with thousands. It's a legitimate MMO - you will be playing on a massive persistent world with no loading screens that makes heavy use of phasing technology to make the world change in response to the quests you complete.
The example I saw was a mining camp filled with crazy folks who've augmented themselves with alien technology. When the group of players approached, they all got given a quest automatically to clean it up. They proceeded to run around, shooting everything that moved. The sound and recoil on the guns felt weak and plastic-y, but this is a very early stage of development. Players can upgrade their gear and their special abilities, which were very reminiscent of the Crysis 2 nanosuit abilities: Overdrive for more damage/armor, Stealth for sneaking, Decoys that you can create to distract enemies, and a teleport. There didn't appear to be any headshots, as an enemy took about six or seven shots to the skull before succumbing to the wounds at one point.
While the developers confirmed that Defiance will be recieving high-res textures and a unique UI on the PC, it was troubling how many times during the demo they used the phrase, "because this is a console game..." I believe they meant to emphasize the action pace of the shooter combat, but it wasn't very reassuring.
The game will be closely tied to a TV series on the SyFy channel that will presumably share the same name. The lore will be shared for both in almost-realtime, with TV show events affecting the in-game world and events in-game possibly affecting dialog or story in the TV show (unconfirmed still).
The developers plan to include PvP, but aren't willing to talk about it yet, probably because they haven't been able to find any amount of auto-aim that balances the playing field when a mouse-and-keyboard PC user is on the loose among console shooters.