Elements of the world
1:Open world with limitless possibility
This gets restricted. You are tied to your friend and neither can go outside of X distance from the other. There would be restricted loading cells.
Even though Defiance is massive enough if you choose to play single player, and Borderlands consists of large play spaces, once you choose to play with someone, you are bound by them. In the case of Borderlands 2, you are free to wander the loaded play spaces as you please, at least until your teammate hits the fast-travel button. In terms of Defiance, you are rarely ever "alone" even when doing a story questline by yourself. As such you are tied to the locations and the actions of others
2:The ability to go anywhere in this open world at any time
You want to go find a rare ring in a certain cave. Your friend already has the ring and says the dungeon is long and boring. So instead of trusting your thrill of adventure and thinking about the fact that every playthrough is different, you skip it. You dont play the game to YOUR wishes and demands, you tailor them to others.
In the case of Defiance and Borderlands 2 this has to do with the quests you choose to do, the events you take place in and the freedom to explore based on your own choices. And sure you "can" do things on your own in defiance, but that would be like playing Skyrim while in a party chat, it fails to suspend disbelief.
3:The ability to do anything in this open world at any time
If you want to save a person... you can... but then your friend kills them. Why? Because that is what THEY want. Sure its fun at first, but the world Judges you based on things like that and eventually you wont enjoy playing as much.
In the case of Borderlands2 and Defiance. Anyone that is an NPC is immortal. Unfortunately in terms of TES, the games seem to be sliding into a similar situation. Essential NPCs are immortalize to protect them from the world whereas many unimportant NPCs lack quests or purpose and some even a name.
4:The ability to be anything in this open world at any time
You may be able to choose how you look... but if your friend plays and is level 40 while you are level 6 either they get stripped or you just hide behind him. The level scaling would become either based to you, based to him... or an average. In either case it is too hard for one player and too easy for the other.
In the case of Borderlands 2 this is exactly the case. In the case of Defiance the player only benefits from their EGO (perk system) but a new player is equally as effective as a player who has played for months (in terms of damage and combat capability). Both of these characteristics are to say nothing towards restrictive player classes, skills and attributes presented by both games. While TES has had class systems, they have never been restrictive, and the player has always been capable of being great at everything, the class system only determined the rate in which the player leveled up in the world.
5:The ability to play the game multiple times and it will always be different
There are always dialogue options, choices, classes, styles, approaches, quests and places you have never been to, and even when you have doing them at a different level is to garner a very different experience. If a game like TES is made for co-op they have to force certain events to happen in order, and they have to restrict the rewards and the dialogue needs to be changed so that it is no longer level specific. In fact they probably would need to remove the skills and levels all together. You just buy better spells and can cast them, or find better armor and can use it immediately. No need to become better beyond equipment.
Unfortunately this isn't the case in Defiance unless you choose a different character class. In fact you can play with your lower leveled friends and beat the same missions multiple times to the same effect. Borderlands 2 allows for upper level playthroughs, where enemies are different names and there are "better guns" but in all reality, it is the same quest, with the same approach and the same outcome. That isn't to forget that quests require playing through the main story in order to unlock them.
Elements of the Player
1:The ability to look and approach situations based on your tastes
Who would get goldbrand or umbra? What if you both... or heaven forbid all 8 of you want the same thing. Either there would be theft and fighting or nothing would be unique. Or you get thrown into a task or role. (Engineer, Recon, Support, Healer)
Both example games have a role system and a specific character model for each. Although some variation and customization is possible, your model is reflective of your role.
2:The ability to choose based on your moral principles
Again: If you want to save a person... you can... but then your friend kills them. Why? Because that is what THEY want. Sure its fun at first, but the world Judges you based on things like that and eventually you wont enjoy playing as much.
In Defiance I can't really think of any moral-ethical choices that need to be made. In Borderlands 2 the only real choice you make is in a side quest as to which side in a family feud you wish to take and even then you sabotaged both sides
3:The ability to accomplish and complete multiple questlines and receive a unique symbol of office or recognition
There cant be 2 leaders of the thieves guild. There cant be 2 Dovakiin. There isn't a prophecy requiring 2 people to save the world. A story just doesn't work and isn't fun like that. Who gets the benefits? The rewards? If its the host, then why would the guest join at all. A questline can take days to complete... just to have to do it again.
In Defiance, anything gained from a questline is equal to anything someone else could get. Arcfalls and side missions have some good loot, but the quality of the gear doesnt matter when the damage dealt and received varies based on the number of people in the area. Borderlands 2 was MADE with 4 players in mind so the Vault Hunters are all the hero... but at the end of the game only 1 person gets the unique head.
4:The ability to challenge ones own patience, limits, skill and feel like you are getting better
You fight hard bosses, you challenge yourself with puzzles and long grueling quests. But as soon as you add another person, you are either waiting on the person to catch up, you are baby sitting them, or you are the one who is hiding behind the other. Only cowards want multiplayer... according to M'aiq.
Defiance never really has the player get "better" beyond their EGO. In the end, some places do require teamwork, but that is only because a player needs others to revive them and because the location was made to make these locations nearly impossible. Borderlands 2 did the same. The "invincible" monsters available to top tier players almost require 4 players. Without them, the creature regenerates to full health everytime you go down. If anything games MADE to be co-op make the players more dependent.
5:The ability to waste time and roleplay as you choose to
You want to go fishing... then go fishing. But your friend wants to go murder a town with a pick axe... one of you has to wait. You lose the role, you lose the down time. You lose the soft little staring into the sunset because you are judged for it.
Defiance doesn't have many of the beautiful elements and Borderlands 2 doesn't give you the choices.
Elements of the game
1:Modding Community
If only Co-op then modding may lead to some bugs, but otherwise modding might benefit from multiple people building a city at once... but then you can lose sight of your vision. It is no longer YOUR masterpiece. Also if a different form of multiplayer ends up existing mods may find itself removed in its entirety... I mean nobody wants to play against cheaters.
Neither game has a modding community to my knowledge.
2:The ability to be a hero and villain at the same time
Again, If a game like this is made for co-op they have to force certain events to happen in order, and they have to restrict the rewards and the dialogue needs to be changed so that it is no longer level specific. Only in TES games can you be hated by the world for a moment and then be loved by them. Some may call this immersion breaking, I call it freedom. They will acknowledge that you are the assassin of legend and they will praise you for saving their city.
Boderlands and Defiance have a single ultimate fate for the player, and nothing else matters.
3:The history is written as if one from the world had witnessed it
Libraries of stories fiction and non-fiction, guides, experiments and politics rivaled by few games.
Defiance and Borderlands have recorders that speak to experiences in the world and give a history to the places and people, however it isn't anywhere near the extent that TES can boast.
4:Every NPC is a personality
Every NPC is different with different things to say, different greetings and different attitudes.
In borderlands they removed choice. You can only kill "bad NPCs". "Good" ones are just decoration or the source of missions. Nobody really "says" anything that isnt related to a quest. In Defiance the only personality is found in the cutscenes, your EGO and the players around you. And when playing a game like defiance or Borderlands, playing co-op can become so satirical and quest-motivated that players pay little attention to the NPCs who do have some effort dedicated to them.
5:Every location/ artifact has a story
Ties into the history, but the story in each location can also be witnessed and tied to a quest giving each area dynamism to each playthrough. This was a bandit hideout where they would trick lumber suppliers into selling them supplies for top dollar, but instead kill lumbermill worker and keep the wood. Or this was a mine that now is occupied by bandits; the original owner died in a tunnel collapse, here is his diary.
Borderlands 2 is good at "making history on the go" with their reuniting of the vaulthunters and revealing of the main story. Defiance has locations that are significant to the series as it is portrayed on TV, but in terms of roads, mountain ranges and the current state of things Defiance can be shallow: This was a medical depot until bandits took over. Go kill them.