Player abilities are earned by upgrading your Base of Operations. As you purchase upgrades, each of your wings will provide skills, perks, and talents. The Medical wing grants you abilities centered around healing or support, Tech is focused on explosives and applying debuffs to enemies, and Security provides options for the defensive-minded player. Skills are active abilities that you can bring into battle - pick two of your favorites and one 'signature' ability. Talents are passive abilities that help round out your playstyle - you can eventually pick four to have active at any time. Perks range from expanding your inventory to adding extra vendors to your Base of Operations.
Tech is probably the most entertaining of the three wings, since it gives you access to all sorts of fun gadgets. My two favorites are the Sticky Mine (modified to spew shrapnel and apply a bleeding debuff to enemies) and the Turret (which can be made to electrocute enemies or even be a remote flame thrower!)
Games like this live and die by the quality of loot they provide, and I'd hate to say it, but this is the part of the game that bums me out. Since everything is (more or less) based on reality, you're not going to get anything that looks particularly cool. The AK-47 you get at level 5 will look exactly like the ones you can find at level 30, except the numbers will be bigger.
That's slightly unfair of me, since higher qualities of weapons and armor also have different perks available, but I haven't really found anything that feels game-changing yet. There's actually more variety in the cosmetic items you can equip - the game throws a ton of jackets, boots, hats (it's a video game in 2016, of COURSE there will be hats), and even scarves.
The game's built from the ground up to be a co-op experience and it's a much more engaging experience when played with others. It'd be nice if you could see other players while you're freely roaming the world, similar to how Destiny or Journey will phase nearby players into your game, but since you can trigger matchmaking at any safehouse or at the start of all of the story missions, you shouldn't have too much trouble finding company.
Co-op is definitely the way to play. In practice, it allows you to be more aggressive, since there are some (very) basic threat mechanics in play. Basically, the louder your gun is, the more attention you can get enemies to pay to you, which lets your partners exploit openings to better position themselves to take headshots, or get in close for a shotgun blast. This also lets you be a little more flexible with the abilities you equip, since you don't have to be as much of a generalist or worry as much about taking self-healing skills.
As more players join your squad, the game will scale things in several ways, one of which is decidedly annoying. Enemies will appear in greater numbers, and often will be wearing extra armor, which changes how you'll fight them. It becomes less of a 'stop and pop' game, and you'll have to avoid getting flanked a lot more often.
Frustratingly, the level of enemies is also scaled up based on the level of the highest player in the group, whether they are the host session or not. The game first checks the level of the zone/mission, and if any player is five or more levels higher than the content, every enemy will be (at weakest) five levels below that player. So if I have a friend who grabs the game later on and wants to play some co-op with me, my level 30 will cause nearly every enemy in the game to be level 25, and that will persist even if I log out.
Aside from the campaign missions, when exploring the city you'll chance across 'encounters' and 'side missions'. Each zone in the city will have its own set of them, and with a couple exceptions, you'll do each type of side mission (uplink repair, supply defense, etc.) once per zone, and at least one of each category of encounter (hostage rescue, checkpoint defense, etc.) as well. Completing side missions will earn you a good bunch of experience and blueprints for crafting. Encounters reward experience and resources for one of the Base of Operations wings. Once you tire of this stuff, I suggest taking some trips to the Dark Zone.