The most "epic" Hitman game suffers from mixed level design.
As before you play as Agent 47 who, wishing to atone for his sins, has retreated to a peaceful abbey in Sicily. He works the grounds for Father Vittorio, the church's sole inhabitant and priest. 47 soon views Father Vittorio as a good friend and mentor but 47's peace is not to last. Mafia kidnap the priest and 47 busts out his suit and harballers, before setting off to rescue his friend.
The tale itself is quite good, though it suffers from pacing issues. It takes along time to see the importance of your seemingly unnecessary globe-trotting, but in the end all is made clear. It has some great throwbacks to the first game and let's just say IO sure knows how to make an ending.
The real meat of the game though is still in the level design. There are many ways to even approach a target, and of course...to kill. One standout mission, in which you have to kill the Hacker Charlie Sidjan, is amazing. Of course, first you have to infiltrate the building. You could strangle a guard and use his uniform to get in. You could start a fire and rush in under the guise of putting it out. You could knock out the delivery boy and wait for someone to order a pizzia.
Other great examples abound. In another great mission, the leader of a Sikh cult needs heart surgery. If you time everything correctly and manage to incapacitate the doctor, you can perform your own "surgery". If stealth's not your forte you could easily kill everyone too, something equally as fun due to the weapon variety and the ragdoll physics. That's one of Hitman's major strengths; player choice.
Unfortunately not all of the levels could be this amazing. The Japanese missions are a slew of frustrating stealth sequences, mostly due to the area's one time gimmick, ID, which renders all your disguises useless. The missions in Nuristan are as equally unimpressive, being extremely small and difficult. Overall the mission design is strong (probably the best missions in a Hitman game), but those two areas left a sour taste in my mouth.
There are other problems that can be heaped on top of that one. The AI is a mixed bag, presentation is poor, and the map is a pain to use.
The AI in this game is pretty clever and it can be downright intelligent at times. If you get to close they will scrutinize your uniform, and in some high security areas they will follow you around, which is extremely annoying (in a good way) to your efforts to be sneaky. If they find a dead naked guy, then they'll know someone took his clothes. So when does it get bad? Answer: When the bullets start to fly. Once combat begins the AI becomes downright moronic. They bunch up in doorways, run at you in a straight line, and don't even use cover. It's common to see a guy open fire and kill four or five guys in front of him. It's also fairly easy to abuse it due to it's deficiencies.
Presentation is rather...lacking. It's not poor, and the story and concept is good which adds to this, but overall the briefings are rather weak. The cool money gimmick from the first game is gone, which is tad disappointing. Also Diana has a bad habit of being rather imprecise in her information, which can make locating your target or stash rather frustrating.
Lastly the map is a far cry from the series' later maps. While it does a good enough job of helping you navigate, and it does list the locations of everyone, it's impossible to tell if they're dead or not. If you're playing this like a shooter and you want to make sure you got everyone, it can be extremely annoying that the map does not note that distinction.
Overall though the game is spectacular. While I may have sounded extremely negative, you really have to play the game to see why it's incredible level design and unique approach to stealth is so intriguing. While the AI is a problem, I would rate the presentation and map problems fairly low, and they are only minor blemishes on an otherwise astounding game. The main disappointment for me with this installment is the mixed reactions to the level design.
I should make one last note. The sound design is very good. Jesper Kyd composed this soundtrack and while it's primarily orchestral, there are some of Kyd's iconic electronic tracks. It has a suitably epic feel which matches well with the scale of the game anyway. This is the most "epic" and massive of the Hitman games overall and it's fitting that the music reflect that.