tldr Defiance is like Tabula Rasa mixed with Warhammer Online that is great for casual players. I think you will enjoy it if you give it a shot.
Defiance is an MMO FPS made by Trion Worlds for X360/PS3/PC that pairs with the show on SyFy channel by the same name. From what I understand, the world of the game will follow the story arc of the show in some way, although I'm afraid I do not know the details.
I guess we should start by asking: what is Defiance the TV show? Well, I don't know either, so that's that. So many shows come and go on SyFy I can't keep them all straight. All you need to know about SyFy is Farscape and BSG, am I right?
From the opening intro, it looks like the show is set in some near future where Earth has become a battleground. Humans have spaceships and there are cat-people aliens that look like Ron Perlman playing the Beast from that Beauty and the Beast show in the 80s. Like most aliens, they seem to only differ from people because they have unusual facial features.
The story in the game is that you are an "Ark Hunter" and you are helping some mad archaeologist/scientist guy, who looks like Lex Luthor or Captain Picard based on what angle the camera is viewing him, find some kind of alien device. It's kind of hard to explain because of two reasons. First, I don't really care. Second, the entire exposition takes place in probably one minute and then your ship explodes and you are left stranded on Earth.
Not such a bad place to be stranded I guess.
So you are an Ark Hunter, and you start building your character by choosing some background like "Machinist", "Soldier", etc. Honestly I have no idea what effect choosing a background has, since it does not start you in a particular place on the skill tree or unlock any special abilities.
Your character has this cybernetic implant with the convenient acronym EGO. Part of this implant is a Cortana-like character that acts as your guide and tutor. Damn what an original feature. I've never seen anything like it. Seriously, do they think I am so lonely that I need someone talking to me all the time and cheering me on? The voice says things like "You're really catching on this quickly aren't you?" and "Great work!" Maybe I shouldn't complain though. After all, it's better than Destiny where that damn robot just trolls you non-stop like your average reddit user.
The game is an FPS (third-person) as I mentioned earlier. The mechanics are pretty simple to describe. You can equip two weapons and a grenade. You have a special EGO-ability, one of either Overcharge (more damage), Cloak, Decoy, or Blur (move faster and do more melee damage). Unlocking an ego ability unlocks adjacent passive abilities on a grid. When you unlock one of those adjacent passives, more passives next to that on the grid will become available for unlock. It's kind of like the sphere grid from FF10. So you get to equip your EGO ability and a couple of passives. As you level up, you can upgrade your passives which will increase their effectiveness.
If you use a particular weapon for long enough, it will level up and get some better stats like bigger magazine size etc. You can also buy scope and magazine upgrades to your guns.
I know that all this sounds fairly standard stuff, but honestly it works pretty well, and I have enjoyed my time playing so far. I think one of the reasons for this is that there isn't really a level system. The passives make you better in some way, but everybody in the game can play together from the first moment you enter the world which is cool.
As far as how the game feels, it is remarkably similar to Tabula Rasa. Assuming you played TR during the 5 minutes it was online before the servers were shutdown, you will know whether you like that style or not.
So what do you actually do in the game? You are thrown into Old Russia, ... I mean, sorry, um, future San Francisco. Apparently lots of strange mushrooms and things grow in the future, so the landscape is not recognizable. You are on the map with everyone else playing the game, and you either go do your main story quest, random events you find on the road, or "Arkfalls", which are like major attacks that require lots of players to defeat. These "public quests" borrow heavily from Warhammer Online (RIP) which was really a trailblazer in this area of cooperative gaming. You can also do PVP, although I have not tried that.
One feature I really like about the game is that if you are working on your quest and some random guy wandering around comes to help you, you both get rewarded. It promotes cooperation and that's a good thing.
I can't say there is really any point to the game except to go around killing aliens and demonic robots and bugs. It's mostly about getting new items and tailoring your character to support your style of gameplay. It doesn't sound that deep, and it's not really, but it is actually surprisingly refreshing and fun to play. You get new weapons and EGO points just often enough that you want to keep grinding for that next unlock or that next cool weapon you might get.