Vertical levels, difficult combat
My first impressions of S:MoS were very positive. Gameplay felt solid and the game itself runs well on the Unreal Engine 3. It is maybe not the most interesting or varying title visually but it is definitely crafted with care. Level design is great as well -- one could say superb even. I doubt there are many games that would compare to S:MoS on map verticality. The sheer height of some of the maps is incredible.
Styx's agility comes in handy on traversing the maps. His well-animated movements reminded me of Death from Darksiders II. Styx is not capable of wall-running like Death, though, even if he jumps well enough. Running in general is a weakness of Styx's. If you get spotted by a near-by enemy, you better hope there is a place to jump up or down to as you will get caught if you simply try to run away from them with your short legs.
Getting away is made even more challenging thanks to the melee combat system. Hostile enemies in melee range will lock you into a duel, in which you have to parry two or more attacks before you get your own counterattack to kill your opponent. And parrying is difficult. Even with a parry skill upgrade I felt pressing the button required very accurate timing to succeed, and camera waving from side to side with your movement does not help.
Platforming somewhat rough on the edges
Ledge detection is good, maybe even too so. Styx grabs onto ledges with satisfactory ease. The problems arise when you need to drop off an edge. If you are not heading straight away from it, Styx might grab onto it and climb back up. This of course happens most often when you are in a hurry.
Also too late into the game I learned how to stop Styx automatically climbing up after grabbing onto a ledge (by holding down spacebar) and how to drop off a ledge to grab onto it (shift crouching). These were told in a seemingly random loading screen tip. Drop kills could be more responsive as well. Executing them was way harder than in Dishonored. Pressing the button would way often do nothing.
Challenging and clever stealth mechanics
Stealth mechanics work mostly as expected. There is the proper three-state alarm system, though sometimes I feel enemies go into the search or even straight up hostile state way too easily when they get a mere glimpse of you. Many actions also seem to make unwarranted amount of noise.
Using the quick takedown kill in S:MoS causes the target to yell and alert everyone in a large area. It is almost never an option if you want to stay unseen. The muffled kill also makes noise but actually sounds more quiet. In Deus Ex: Human Revolution you can use non-lethal takedowns very close to other enemies without alerting them even though they sound about as loud as the lethal ones that supposedly make more noise.
Enemy variation is pretty great for a stealth game. Each of the seven missions introduces at least one new type. The most troublesome of them are probably the knights whose full plate armors make them immune to takedowns and throwing knives. Instead you have to poison their water bags or apples with Styx's puke, or simply drop a chandelier on top of them.
Modest amount of abilities and items
Styx's abilities have a number of upgrades. They are generally not too exciting but have their uses. It was difficult to decide which ones would be the most important to get. The equipment tree might have the most useful abilities. Being able to carry more stuff makes things easier.
Each mission consists of three to four large maps. Unfortunately S:MoS reuses the maps quite often. Nothing changes on them apart from different enemy placements. It gets fairly dull to see the same locations multiple times as for 100% completion you need to play missions twice to get all the four insignias for each. Some forum posts claim you need three playthroughs but I found two to be enough. Maybe one could be enough but that sounds extremely difficult if you are playing the game for the first time.
40 hours for a completionist
On my first playthrough of a given mission I went for Shadow (no enemies alarmed) and Thief (collect all tokens). Shadow is fairly lenient -- only you being seen counts for it. Bodies discovered, prisoners freed, or clones being spotted do not matter. Thief on the other hand is maybe the most annoying one to do. Every map apart from the very first has exactly 10 tokens (which feels very gamey). Finding them without a guide would have taken ages.
On my second runs (completed missions can be replayed from the hideout) I went for the remaining Mercy (no humanoid kills) and Swiftness (complete the mission within a time limit) insignias. The latter is not too strict and no one should have trouble getting the insignia. And if you screw up, you can always reload the mission, zone, or a quick save -- for once a game that allows you to save without stupid limitations!
Writing is Styx's weakest side without a doubt. Story and dialogue were pleasant to follow at first but as the game progressed, both turned stale. Lines became meaningless, way too long lasting ramblings. I started wishing Styx would just shut up. The modern profanity also feels really out of place. The Witcher series is probably the only one that can get away with it in a fantasy setting.
Regardless, Styx: Master of Shadows is definitely the best stealth game I have played since Dishonored last year. A sequel: Styx: Shards of Darkness was announced recently. I guess I might check out that one day. The developers have promised a bigger budget, bigger ideas, and a new engine (UE4).