I’ve played quite a bit of helldivers with my brothers, it’s quite good as a coop game.
It’s a bit like Magicka, in that in the chaos, you most likely will hurt your mates if you are not careful, you can accidentally(?) squash them with the items/vehicles you call in from above or you can hit them with an air strike or your guns.
I’d recommend voice communication in some way, we usually default to Skype, so I don’t actually know if Helldivers got built in voice comms.
There are some interesting coop interaction, like some air drops containing several items that you can share/get stolen by your teammates (a good example, ammo drops come with a few packs, but one pack is not enough to fully fill your ammo supplies, do you share or max your own ammo, will your teammate even give you that choice?), a rocket launcher that works best by having another player carrying the ammo pack and reloading you after each shot, vehicles, missions to carry items that limit you to your sidearm so you need to rely on your teammates and so on.
The game has an interesting air drop system that works a bit like Magickas spell system in that you need to use a combinaton of buttons to call them in and memorizing the right order helps alot. Writing the correct order for an air strike can be quite the ordeal when the enemies come barreling down on you, so that can be quite intense.
Ressurecting each other is quite easy, with a short cooldown, as long as one player is alive, they can bring back everyone else in a few seconds (if the drop is not on cooldown) but once again, sitting still and write the right simple combination that is prominently viewed on screen while the enemies are closing in and your teammates are depending on you, ONLY YOU, YOU CAN SAVE THE DAY RIGHT NOW can be a bit daunting at times. That being said, the times when you are the last person alive, get ripped apart by an alien, and everyone goes silent for a split second before they notice the glowing air drop beacon falling to the floor is quite great.
I think the game would grow old fast if played with people that actively try to squash you/shoot you in the back, but when everyone works toward winning and accidents are quickly forgiven it’s a fun, chaotic mess with alot of difficulties that works great in coop.
Edit: The gameplay is very close to Alien Swarm but on big, more open, randomized maps, with several random objectives scattered about. Alien Swarm is free though, so maybe try that out first to see if you like roughly the same gameplay.