Arkham Knight starts off with Joker's cremation. With Joker gone, Gotham City has a period of low crime. However, this wasn't going to last forever, and Scarecrow attacks a diner and threatens a large-scale attack. Gotham is evacuated, leaving Batman with the police and criminals.
Batman now has a Batmobile which is used for navigation, combat and unlocks new areas using its winch. It's more like a heavily armoured tank. It can drive fast, but often travelling using glide and the grapple is even faster. You can transition in and out of the Batmobile with ease, since you can launch yourself out of the Batmobile into a glide, and can be summoned to your location when landing, or on land. Some people see the vehicle's inclusion is constantly trying to be justified by forcing you to use it in some sections. You can use it to destroy walls with the gun, pull down walls with the winch, create makeshift ramps with the winch, and supply power (again with the winch) to progress to new areas. Batman can also remotely control it which is used to get yourself out of tricky situations.
In combat, the guns will automatically detect if you are aiming at a human in order to fire a non-lethal round instead. When against vehicles, you are against unmanned combat drones, but then there are some other car chases which do seem to be manned so there's probably some deaths somewhere. The Batmobile can strafe in "combat mode" and the drones telegraph attacks with a red line of sight, so you just dash away from the lines while lining up your shots. Many people seem to hate this combat mechanic but I quite liked it since it varied the gameplay. The criticism I would have is that it is used more in the last few parts of the story and the difficulty does ramp up. This means that you do have to get good with the mechanic and probably fully upgrade your Batmobile's armour and weapons to help you out. There are a few types of drone, with the ones that fire a missile barrage being particularly deadly. There's a special type of drone which plays out more like stealth/hunt missions as you try to manoeuvre behind them for their weak point.
Batman gains a new suit pretty quick in the story. The suit, and Batmobile seems very Transformer (film) like with the way it is constructed. The overall presentation is very film-like and you can draw some comparisons to the Dark Knight series. The graphics are very impressive and it is always raining which does give it a more interesting look. There are some nice details to notice like the rips in Batman's cape.
There's the usual travelling and busy work that you get in open world games, but I felt this was more enjoyable in this game. The grapple mechanic is extremely satisfying to use, quickly vertically scaling buildings. Horizontal movement is pretty fast too because you can transition into a glide or use the grapple to traverse between nearby buildings if you wish. It also leads to satisfying combat mechanics with stealthy/stalky gameplay. If you have played the previous games you will be familiar with the combat sections.
The basic combat is where you can take down many enemies quickly with simple button presses. Combat sounds easy, but when there's different enemies, then you have to put more thought and strategy into it. Knives can be countered with holding the button and moving away rather than tapping the counter button, shielded enemies need a stun and jump combo, the strong enemies need to be stunned then pounded, shock troops need to be attacked from behind.
The other type of combat section is where you are trying to stealthily take out patrolling enemies. You can use your gadgets and environment to create distractions or take them down in creative ways, or use your stealth attacks like hanging down from a gargoyle statue, popping up out of a grate, or silently taking them down from behind. There's some scope for more direct attacks, especially when you can smash through glass onto them. Even if you are spotted, you can easily flee with the hook shot and grapple between Gargoyles or over high-ground objects.
The ending of the previous Arkham games got a bit weird with the Joker turning into a monstrosity. In this game, his blood has infected people which has slowly turned them into Joker. It also causes Batman to hallucinate and often see and hear The Joker who mocks him. The Joker's dialogue is amazing; there's some incredibly funny scenes.
There's a series of different side-quests which will end with you taking down the likes of Penguin, Two Face, Firefly, Riddler. Arkham Knight is part of the main quest, and Poison Ivy will assist you. You get to fight alongside Nightwing in the Penguin storyline.
The Riddler still has his Riddler Trophies scattered around Gotham. Just like the other games, I only bothered grabbing a few of them, but other people really like this mechanic. He also has some Batmobile races (which are apparently VR) but it's not exactly a "riddle" so not sure what he's thinking with these! These courses you have to activate gates with a button press so there's some timed presses whilst racing. He also has a series of challenges that you need to complete to rescue Catwoman. She has a bomb collar which requires 8 keys to take off. Maybe he has been watching the Saw films recently.
There's some interesting missions along the main story, and I did really enjoy the one early on when you visit ACME chemicals. It is split with multiple objectives and you have to work out how to get the Batmobile into certain locations in order to progress. This involves wiping out enemies in different locations, having to rescue workers, and repositioning a bridge.
Not sure you can do many side missions without progressing the story. Eg Part of the story you will lower a bridge which allows you to go there with the Batmobile. This restricts some of the missions; Riddler's storyline in particular. I'd probably just advise focussing on the story to around 70% so you don't run into that block. You do need to complete loads of these side-quests in order to actually finish the game, but there is a daunting amount of them. Apparently, you get a slightly modified ending if you get all the Riddler trophies as well.
The game was infamously glitchy on the PC release, which caused it to be removed from Steam. I thought it played flawlessly and didn't come across any bugs at all. Even the loading times were impressive, and once you are in the game, it is completely seamless (apart from the massive lifts in a couple of places, mainly used for the Riddler Races).
Like many open world games, there's too much content to play 100%. I think this game is one of the best for navigation, gameplay variety, and presentation though. Many open world games bore you with tedium, but I think the gameplay was varied and the difficulty scaled to constantly give you a challenge.