X-com 2 is the sequel to X-com Enemy Unknown and its expansion Enemy Within. In this version, you failed the first game and the aliens won. Now you are leading the resistance against the aliens and must try to take the Earth back.
Lets start with what X-com 2 does well. The graphics and animations are much improved from the original games. The art design is cool and it's a treat to look at all the alien cars and buildings, among other things. Also, the procedurally generated maps ensure you'll likely have a different experience every time you play.
There are also new weapon mods that allow you to modify your weapon with things like increased aim and critical hit. There are also combat sims that give your soldiers a bonus to will or focus, for example. These items occasionally drop off of killed enemies, which adds a cool layer to combat, because if you go crazy with explosives, you could destroy an item, along with weighing whether you can spare a turn to grab the item. Thankfully, there's a new resource called intel that allows you to buy some of these items on the black market as well. Oh and you can even add weapon decals and colors if that's your thing (you'll have to redo it once you upgrade their weapon though).
There's also a hacking side of the medic class now (specialist in X-Com 2), but I found it to not be very good as the chance to perform the hack were very low. It is nice that you can hack objectives from range (with a usual 100% chance), but other than that it's not to useful. The specialist class now has a mechanical robot that flies around them and can heal (from a looong range), defend, scout, or even damage enemies and many actions don't end your turn, so the class is a lot better than in X-Com 1!
The Ranger class (assault in the original game) also received a change in the form of a melee weapon, along with their standard shotgun. I'm not sure if the melee weapons were added last minute or what, as they're super strong. You can move your full movement and end with a sword slice that will 1-shot most early enemies. Certain later enemies will be better prepared to deal with melee, but the hit rate (usually 88% or higher), range (you have your whole turn movement range), and damage of melee attacks make them super strong, not to mention the +2 damage to melee attacks perk you can get very early.
Other than those small changes, the game is extremely similar to the original game. Many things received new names, but preform a near identical function. Even the glitchy camera returns that sometimes makes using the hacking data pad annoying as a wall will block the hacking screen. The story cinema parts are the same, just different dialog reaching the same conclusions as the first game. The enemies, while looking different, generally do the same things. Don't expect very many new experiences.
Don't expect a great story (as in the character development is minimal), as is tradition. This time, you're trying to stop the aliens from completing a project and there is an annoying forced countdown to the end of the world. The timer can always be extend, of course, but it feels annoying to always be rushed or to be forced to stop exploring something to go deal with a threat the game won't let you ignore. I wish they took a different approach.
There are also a lot of small changes that I did not care for. You now have to get your monthly income from "supply drops" by scanning it over 3 days to get your full income with your ship. You'll also have to locate the black market, as it doesn't start out available right away like in the original.
The really increased injuries in this game, to a ridiculous degree. Many soldiers can take 20+ days to come back from injury. You'll have a huge list of injured party member you can't use. Also, 99% of the time your people will die in combat, as the option to stabilize almost never comes up like it did in the original game. If all your Rangers are injured, the early-mid game becomes much harder, as them and specialists are very important. You can train rookies one at a time in the school to be what class you want, but it takes at least a few days.
Injuries wouldn't be a problem if the aliens weren't so tough, but they are. Every enemy has received some kind of upgrade and it can be ridiculous, even on veteran. If your top people are injured when the tougher enemies start to show up, you can have problems. Due to the way the game scales, your pigeonholed into a certain way of playing, instead of playing how you want. To make things worse, many mission have a turn limit or people saved amount, either way there's usually pressure to go faster than you would like, leading to more injuries for your squad.
The procedural generation comes into play here in a big way - sometimes you get a really easy roll (the vip spawn in your group and the extraction point is really close) and other times the maps can be close to impossible. Also, it's very easy to chain-pull 2-3 packs of enemies in one turn, which is very hard to come back from. I think the map generator needs some more constraints to make a better experience for the player. You'll get used to reloading a lot, but it gets a lot easier once you get certain tech and buildings.
Essentially X-Com 2 is more of the same, for better sometimes and worse other times. Since it's Firaxis we know we'll get regular patches as well as mod support, so the score reflects what this game would be once some issues are fixed. Right now it's more of a 5/10 (average, very average) for me personally, but if you love X-Com, you'll still probably enjoy this quite a bit.