Few concrete gameplay details have been released since the game's announcement, but today the developers did let a few extra details slip.
- Two player online co-op is confirmed for consoles and PC. Console players will also have the option of split-screen co-op.
- Co-op will be an entirely separate mode that will not include the story elements of the single player campaign.
- Hooper mentioned "armor" and "inventory" systems, and indicated that there will be some type of world economy that the player can participate in.
- The shooter gameplay will play out in the tradition of id FPS titles.
- Vehicles are being used as an "extension of the player," allowing users to drive to instances, or race on tracks that are found in each town.
- Racers will have two seats for co-op riding, with one player manning a gun.
- Vehicles can run over monsters, but the gore will be limited, as id is shooting for a Teen rating to broaden its audience.
- Training missions played at racetracks will teach players new vehicle skills.
- Though the world will be large, it is not a sandbox title or traditional "open world" game--players will need to complete the game's story quests in a linear fashion.
- Instances, encounters and items will be crafted, not procedurally generated as in a game like Diablo.
- The game is targeted for release sometime within the next two years--likely in the final half of 2009.
Carmack also took some time to elaborate on how the team is approaching the weapon design:
I really do think that on past projects, I've made these lists of, here are all the possible things that a weapon can do, visually, audibly, motion-wise, all of that. Now it's like, how do we want to apply this to each different weapon to make them work well. Because weapons are still one of things that as you survey first person shooters, there's this clear distinction between the "A" titles, and the "B" titles that just don't do their weapons very well. They don't look good, they don't feel good--they just don't come off nearly as well. So we're definitely gonna make sure that that aspect of it--the tactical, low-level [aspect of] the weapons--will be cool. They'll have their punch, they'll be fun to shoot, and you will have fun as you go around firing your weapons at the bad guys.
"Co-op will be an entirely separate mode that will not include the story elements of the single player campaign. "
I figured id would have the resources to you know, make storyline mode have co-op. Every developer that bails on it seems to site resources and complexity, and that the single player campaign will be more exciting as a result. blah blah blah
Co-op is great stuff, why do developers consistently abort?
Unfortunately,I think you pretty much answered your own question; time and resources. In addition to just simple game mechanics, you have balancing issues, story direction, and countless other reasons why co-op just simply is a reasonable aspiration for any heavily scripted FPS style action or RPG. Imagine co-op Mass Effect or co-op Half Life 2...
Take your favorite movie, and take a long look at the main character. Now, remove the main character, and replace him with two separate, distinct people, with their own personalities. Assume that I will digitally remove the previous main character, and CGI in the new characters, free of charge. Now, figure out how much of the *rest* of the movie you need to reshoot anyway, because everyone *else* in the movie is talking to one person.
Remember, the goal is to make both main characters important. If you make one of the two characters important, and the other ignored, you fail.
Even if you throw out the story concerns, there's significant engineering concerns. Scripted events can no longer rely on assuming the player is at point X, since player 2 can be at any other point Y. Games that allow join-in and drop-out multiplayer (like Quake) have to decide what happens if someone quits while carrying an item needed to continue.
Trying to compromise on all fronts gets you the original Neverwinter Nights campaign. Trying to make the singleplayer game without worrying about how it'll work in coop gets you Hordes of the Underdark.
In short (too late), developers consistently abort because there are real problems, questions, and design decisions that aren't easily solved. I honestly think most people have too much pride in their work to make it only-technically possible to play SP in coop, and ask their fans to completely ignore the fact that Alex is crushing on the Four Gordon Freemans.
Cost/benefit analysis doesn't pan out.