It speaks volumes about the overall social experience in an MMOG when the specific term "multiplayer" is coined in-game to describe PvP matches. This distinction only compounds the fact that Defiance feels far less like a true MMOG than it perhaps should. Naming conventions aside, multiplayer modes come in two distinct flavors in Defiance which are perhaps one of the game's strongest points outside of major arkfall events.
Team death match modes play out on custom maps and offer a very direct quick match option to allow players to hop in and out of PvP relatively quickly. Thankfully matches have become much more stable since launch, so the experience isn't marred by constant disconnects from the server as it was in our first dozen or so attempts to join. The Shadow War is where multiplayer modes begin to shine, making great use of existing landscapes from the core PvE experience. Shadow War is essentially a large scale point capture scenario, and lends itself well to the use of your full arsenal of weaponry, skill, and perk loadouts.
As noted previously, a main pitfall to the multiplayer side of Defiance is the lack of proper social tools in-game. It also doesn't help that core social UI elements such as friends lists or clans are buried a few layers deep in out of the way pockets. The chat interface is also practically non-existent to the point where I've only seen text chat from other players as few as three times in two solid weeks of playing. It's also an oddity that you don't even see the usual drive-by group or clan invites that you normally would in an MMOG, but then again Defiance also lacks any true social hubs where players can gather and socialize on any level.
In the end, Defiance generally does play more like a very large single-player game more than a true MMOG. Were it not for the massive arkfall events you'd rarely get the sense that you're on a game server with more than a few other players at any given time, so the multiplayer aspects of Defiance are in much need of attention if Trion hopes to retain an active player base long term.
Defiance attempts to pack a decent amount of value into the cost of the box or digital purchase price. It's not the type of game where you'll ever feel the need to rush headlong towards a level cap or distinctive endgame progression, so you can easily while away countless hours zipping around the countryside running missions, challenges, or completing chunks of the main story arc.
That's not to say there aren't any overarching goals or carrots for you to chase on any given character. For example, a series of contracts serve as daily quests in Defiance in a similar way to the daily achievement system in Guild Wars 2. Completion of contracts will help you progress with one of the five factions, and net you some currency specific to each that can be used to purchase higher quality weapons.
While Defiance does offer two basic options for your character's race, I still haven't stumbled across any real reason why you'd ever want to create more than a single character. All that would really help you accomplish is the chance to play through the very linear main story arc a second time, as there really is just the one path through the game.
An overarching sense of "been there, done that" kicks in very early on in Defiance. Secondary missions all begin to blur together to the point where you can predict exactly what will happen during each before arriving at objectives. The main story does offer a bit more variety, but even then you'll encounter recycled instanced locations more often than you should considering how little instancing is actually used in the game.
The lasting appeal for Defiance will largely boil down to how cleverly the story of the television series is interwoven into the game. For example, leading up to the season premiere players could play through a short mission chain which involves the main characters from the show and a particular item which turns up in that first episode. A regular stream of events or missions of this type would go a long way towards breaking up some of the monotony of the game, but it remains to be seen just how regular these will be deployed moving forward.
Defiance would have ultimately made a lot more sense as a sandbox RPG / Shooter hybrid along the lines of Fallout 3 than attempting to be a fully featured triple-A MMOG. Due to the awful PC user interface and lack of social mechanics it ultimately does play a lot more like a single-player game than anything else.
The gameplay does tend to grow on you over time, but it lacks that initial dazzle factor needed to encourage you to stick around after the first few hours. If you stick with it, you just might find yourself contemplating loadouts and zipping around the countryside in hopes that another major arkfall event might kick off nearby. At the end of the day, however, Defiance is a pretty average game that may only survive the test of time based on the popularity of the TV series.