A fully realized game world.
If I had to choose between the dense, detailed, and populated open world of Arkham City or the sparse, sprawling tundra of Skyrim, I'd pick Arkham City any day. Every crevice of this game feels detailed and expertly designed. It's "small" by open-world standards, yet still an expansion from Arkham Asylum, but no area is left for randomly generated ornaments to fill the scene or copy-paste dungeons. It all feels lovingly handcrafted with puzzles and side-quests littering any square foot that isn't relevant to the main plot.
Picking up after Arkham Asylum with a slightly ridiculous plot and setting as an excuse for Batman and almost everyone in his immediate canon to appear in the same place, the game kicks off with the type of exciting, innovative gameplay and design choices that made its predecessor so memorable. Swapping between Bruce Wayne, Batman, and Catwoman you encounter many familiar foes and antagonists that constantly stir up the plot - enemies appear more and less important in the grand scheme of things than initial impressions would assume allowing for characters to easily weave in and out of the action, surprising you with each new turn. The Joker remains a central antagonist, but with a much more illusive, and compelling motivation than the typical stock plot, as in the previous game. A mysterious illness threatens his life, making him into a warped Wizard of Oz whose reputation precedes him, and voice that rings through the speaker systems of this city prison. Is Joker even still alive? These are questions you, and the many mindless thugs wonder aloud as the game progresses. The Joker casts a large shadow over the game, but you wonder if he's become like Batman, a symbol of fear. Keeping Joker behind the curtains allows the games array of other villains to come forward in fun and interesting ways.
Striking a sweet balance between the combo-heavy combat of God of War and the stupidly simplistic Assassin's Creed games, Arkham City's combat is fast, simple, and satisfying. It's a simple matter of pressing two attack buttons but the game regularly swamps you with an overwhelming number of thugs at once, forcing you to get a rhythm to your attacks and think one step ahead of the enemy. It's a terrific and simple system that makes you feel like a superhero, predicting and anticipating attacks and pre-emptively countering or striking first. As you progress, the game throws more variables into the fights: armoured enemies, shielded enemies, armed enemies. These change the dynamic significantly which is countered by steady and straight-forward combat upgrades which introduce only an extra button or two to combat, which seems simple, but in the midst of a fight, remembering how and when to execute a specific move can drastically alter the outcome of a battle. It's a system that looks simple and sluggish from the outside, but is constantly rewarding and adapting for the player.
Much has been said about Rocksteady's outstanding ability to finally make the player feel like a superhero, which has somehow eluded nearly every superhero game developer ever, and it is completely accurate. Arkham City may pile the gadgets on a bit thick for players who didn't familiarize themselves with the last game, but it uses each one with consideration and sparingly enough to keep the variety fresh and exciting. From detective work to soaring through the city, Arkham City achieves the toughest goal facing developers today: it creates a deep gameplay system in a rich, controlled environment all while making it look and feel extremely accessible.