While the usual elements are there -- climbing, sword fighting, guns, blow darts, smoke bombs, etc. -- there are three new additions that spike up gaming enjoyment levels, namely: sailing and sea combat, deep sea diving, and the clever meta.
Here's a scored summary of the game's many excellent features:
Kenway can fight with two swords, shoot enemies in quick succession with up to four pistols, put targets to sleep or make them go berserk using darts, and use smoke bombs to escape (or to wantonly murder the coughing, helpless guards, as one does). As a pseudo-Assassin, he can perform covert kills from up high or hanging from a ledge. He also gets a rope dart in the game's final act, which can be used to string victims from trees as a gruesome finishing move.
The AC series has been getting better at making sneak kills easier, so I was actually less openly aggressive in AC IV, preferring surprise attacks from dense foliage to my previous run-in-and-stab-'em-in-the-gut approach. This seems more in line with the Assassin way.
My only beef with the combat system is my inability to control who Kenway goes after during melee combat. The AI auto-targets the biggest threat, so whenever I knock away a bad guy, I often can't go after him for the kill because the AI will then have Kenway attack the jerk who's hovering right beside him. This gives the other guy time to recover, so then Kenway remains surrounded. Argh!!!
Other than that, fighting was a joy, especially on the high seas. Players can steer Kenway's ship and use broadside cannons, mortar, or chain shots to sink or incapacitate unfriendly ships. Players can board hapless ships for their booty -- which means engaging in combat on the deck -- or just send them to Davey Jones' locker if the take isn't worth it. Later on, boarded and plundered ships can be added to Kenway's fleet, one of many diverting sidequests, leading us to...
Oh gosh, where to begin. The sidequests really pad out the game's running time, because there's so much to do! Let's start with your pirate ship. Apart from playing a key role in traveling throughout the world of AC IV, later on you can pimp out your ride with some sweet sails, mastheads, and steering wheels, but it would be wise to get its armor and weapons upgraded first. This is critical if you want to sail the seas unmolested.
In AC IV, Kenway goes island-hopping as part of the main quest, but adventurous players are also allowed to, say, go after an out-of-the-way fort to raise the black flag there and discourage scouting enemy ships. There are treasure chests and Abstergo fragments to collect, wildlife to kill and skin to make new equipment/outfits, and naval contracts to complete. There are also Assassin's quests and Mayan stelae artifacts that unlock really cool armor for Kenway.
Kenway can go deep sea diving to retrieve plans that help upgrade his ship, or dig up chests using found treasure maps. He can open new bars on islands by getting into fistfights, attack royal convoys for cash, infiltrate gangs' lairs and take their bounty, and go out on a rowboat to spear sharks and whales. It's all loads of fun.
My teensy complaint is the main mission bits where Kenway just has to walk with a person to a specific destination. Gawd, add it to the cut scene already!!! I can get acquainted with the new location just fine on my own, thank ye very kindly!
Plenty of pirate-y themes for action and tension! The voice acting is also pitch-perfect.
The plot is pretty basic -- man wants thing, gets thing, loses friends along the way -- but it's saved by the fact that Kenway is practically an accidental Assassin, and by the meta added to the AC series' usual story-within-a-story premise. You, the player, are a research analyst diving into the genetic memories of "Sample 17" to gather footage (i.e. Kenway's adventures) for a pirate-themed video game being developed by the entertainment arm of the Abstergo company. As the analyst -- and the player -- take time out of the Animus and wander around the Google/Facebook-like company headquarters, the modern-day Templar's connections become obvious, while the Assassin's fingerprints are equally clear. What's funny is the inclusion of the Abstergo logo in the actual AC IV opening, and the incredibly cheesy opening scenes created for the game that you're supposedly helping to develop.
It's hard to top the pope as the Templar big bad for an Assassin a la AC II, but Kenway gets to fight a decent procession of straight-up evil mofos as well as morally ambiguous men and women. The final boss is laughably easy to defeat -- it's the getting to him that's tricky. I score this game element lower because all the bad guys are predictable, but add a bonus half point for the wild card villain -- you'll know who it is when you play.
The ocean, with its sunrises, sunsets, storms, and rogue waves, is always a delight. The numerous islands are varied in appearance (except for the ones with chests or fragments, those all look alike), and underwater diving is terrifying because my two greatest fears are sharks and drowning.
But what really stands out in AC IV are the facial expressions of the main characters, especially Kenway. There's one particularly emotional scene where his anguish is captured perfectly. Animation sure has come a long way from Sid Meier's Pirates!
TL;DR: PIRATES!!! No other reason needed.
This post brought to you by 25-mph winds! Ahhh, springtime!