Give me my affinity, alien scum!
Item wise the game is very grindy unless you like spending money in the cash shop for platinum, however that isn't really a problem since the game play is super fun. You can play it like Metal Gear, with patience and silent killing all the way, or just go loud and slay everything - be it methodical and purposeful or a fast paced maddening sprint. The maps are sprawlingly huge, but fortunately as your characters can move pretty fast. Random elements also keep you on your toes, like a rescue mission suddenly evolving into a data collection task - or having an evil warframe appear out of nowhere and begin hunting you down. As to be expected the enemy fodder is limitless much like the zombie horde in Left 4 Dead, with baddies literally coming out of every orifice of the map at some parts.
Important to note: you are just slightly less fragile than them, which makes dying a real concern because if your allies can't get you up before you bleed out, you are out of the game unless you wish to self-revive (which you can only do four times a day). Good thing finding online assistance is really easy, even if you have no friends on (or just have no friends). Anywho, if you enjoy co-op shooters you should probably give Warframe a look. It's not that big a download and more importantly, it's free! :)