The multiplayer get's even better, but little changes regarding single player.
The campaign is quite short, but what little there is offers a decent amount of variety and overall is is quite fun. The player goes from defending a pipeline in Alaska, battling through a suburban neighborhood in Nevada and subsequently fighting to reclaim the Hoover Dam, assaulting an airport both in the air and on foot, to battling through battle-scared city streets in a Russian tank. Along with the tank mission, on multiple occasions the player assumes the role of a gunship providing cover fire for a team making it's way through the landscape down below. These levels go a long way towards fostering variety, and preventing the action from getting too stale or repetitive. There are some issues with these levels, however. Most notably the tank level. When there is a lull in the action, the level can feel really dead and lifeless, as you're driving through portions of the city with absolutely nothing going on. Not only that, the level of challenge in that level is a little suspect. At no point did I feel particularly threatened or near death.
That then brings up the issues of the AI's utter lack of any real intelligence, which can tend to be very frustrating at times. The enemy AI makes itself a sitting duck, simply crouching in the wide open and firing at you, allowing you to easily mow them down one by one. While you can get away with that on the lower difficulties, it isn't that simple on Hardened. While the AI remains astoundingly stupid, the increased amount of damage that you take with each shot requires a little bit of patience and strategy to take down the enemy/s standing between you and the next area.
Technically speaking, past the AI issues, the game is absolutely impressive. Animations look nicer for some returning guns (M40), the scenery is fantastic, and the amount of activity that goes on during battle such as jets or helicopters flying by overhead is a nice touch. To further emulate the console experience and add further production value and quality, n-Space made the addition of of bullet time events that can be found interspersed throughout the campaign. These events are where, after a door gets blasted open with a charge, you and your team enter the following room or area in slow motion. As has become an expectation, the game is impressive aurally as well. Oodles of voice work accentuate and add to the action, and the music is suitably dramatic when in tense and close-call situations.
With Black Ops last year, n-Space made great strides towards improving the multiplayer experience with the addition of features such as perks, join in progress, and the ability to freely select your secondary weapon (confined just to a pistol in previous installments). Defiance continues improving the multiplayer experience, adding a couple key new features and refining a couple others. The two primary new features introduced by Defiance are macros and load outs, allowing you to preset your preferred match set-ups and weapon and perk configurations. Another notable change is the ability to customize availability of certain weapons and perks, rather than simply having to choose general match parameters like "Pistols-Only" or "No Explosives" the host now has complete control over what weapons the players in the match use. This proves to be a significant change, as if there is a specific weapon that particularly bothers the host (shotguns immediately comes to mind), they can disallow that weapon. Further refinements can be found in a new tier system for both weapons and perks. Previously in Black Ops, a player could assign any weapon and any perk to either slot, which would often lead to incredibly overpowered set-ups (such as dual assault rifle and the perks Stopping Power and Body Armor). The tier system lends much needed balance, limiting the player to carrying around only one powerful primary weapon and preventing the exploitation of multiple high-powered perks. Not only does this give much needed balance, it forces the player to really strategize and put more consideration into their set-up.
Along with those general improvements some newly introduced modes (Gun Game, One in the Chamber, Sharpshooter) provide a fun twist on the typical run-of-the-mill deathmatch. The real highlight proves to be Gun Game, a race through twenty levels, each level represented by a weapon and a kill with each weapon progressing you on to the next level. The inclusion of the ability to decrease your opponents level by knifing them adds a layer of strategy on top the mayhem.
Overall, Defiance can be seen as a step forward in one regard (multiplayer) and as a step back in another (single player). Clearly to make way for the multiplayer improvements, n-Space really cut down on the variety of single player modes. Where Defiance has only campaign, Quick Play, and Challenge Mode, Black Ops on the other hand had Arcade Mode, the Killhouse, and Zombie Mode in addition to those three. It is understandable why those modes/features don't make a return in Defiance, but that doesn't mean that it isn't frustrating. When it's all said and done, though, the multiplayer and the replay value that it grants the game is what truly matters.