First off, the game is a top-down shooter, meaning you and your buddies are looking down at your little characters as the enemies come rushing in from all sides of the screen and you have to shoot them before they kill you, be it by claws or bullets. But instead of jumping into a preset level and killing all the stuff in your way, beating the boss at the end, Helldivers takes a different approach. There isn't a boss at the end of every mission, and there are specific objectives you have to complete when you appear on the map. It can be a number of different things, and often you'll have to complete more than one objective before you're allowed to extract from the zone and complete the mission.
But the single most important thing about this game is that there is friendly fire. This game is incredibly unforgiving and everything that could realistically kill you will. If your friend shoots an enemy and you are between him and that enemy, you're dead. If you place a turret down on the map to shoot the bad guys, and you run in front of the way, you're dead. If you call down some more ammo and stand on the beacon you placed, that ammo is going terminal velocity, dropped from who knows how high up. You're dead. If you reload your gun without having shot all of the bullets in the magazine, all of those bullets are now gone forever, because you just wasted a half-full magazine.
Another huge challenge in this game is that in order to do a lot of things, you have to put in a specific set of commands. If you run out of ammo, you have to call in a resupply, and in order to do that you have to hold the left bumper and hit, in order, Down, Down, Up, Left. In order to revive a teammate you have to hit Up, Down, Right, Left, Up. If you mess up, you have to start all over, and trying to hit those buttons really fast while being shot at is no easy task. It can be pretty stressful when your whole team is relying on you to do things perfectly.
This game is serious business, and it gets out of hand very quickly. It requires a lot of coordination with your teammates, but even then some things will happen that you could not have prepared for. There are quite a few types of enemies, so facing each for the first time will definitely kill some friends. But in the end, it also has a leveling and upgrading system. So when you land on the planet surface, staring your second mission, you can pull out your shotgun that much more quickly because you spent your hard-earned research points, and, if you so desire, you can kill your little brother even more dead than when you shot him the first time. Not that I speak from experience.