1. Stealth - at its core this is a stealth game (imo second favorite only to Splinter Cell Chaos Theory). And it gets stealth right. It doesn't "lack" a combat system. There is no combat system because in real stealth combat means "game over".
2. Level design - an integral part to stealth is the freedom of approach. An integral part to the freedom of approach is level design. This game works because of its intricate levels. Always more than one way to skin a cat in this game. If you get stuck, there's always a shortcut or a passage that you missed.
3. Gameplay - incredibly intuitive and leaves plenty of room for improvisation. Once you get used to how things work there is nothing you can't do. No glitches. I haven't experienced a single glitch in my so far 50 hour playthrough.
4. Interface - not cluttered. Pretty straightforward. Nothing vague or misleading. "Less is more" through and through. Prompts for Amber powers appear as soon as you press the corresponding key as to not interfere with your field of view (in stealth games FOV is very important). Throwables and consumables fade out. HP bar is always there. Kind of useless if you ask me. If you're playing the game right your HP bar is always full. Doesn't have a quick load shortcut key on the controller, but they changed that in the sequel.
5. Replayability - of course. You can even replay missions within the main campaign thanks to an option in your hideout.
6. Story - it's almost too clever.
7. Style - cutscenes are without animation, but they're really beautiful pictures of the characters (imagine Hitman, but better looking).
8. Performance - it can run fast on incredibly low-end PCs and can look pretty good on high-end PCs. Not demanding at all.
9. Atmosphere - everything from visuals, to soundtrack, the overall setting is executed perfectly in my opinion.
10. Details - as Sherlock Holmes often says, details are the most important. An amber mark on Styx' back and a subtle vignette effect let you know when you're in the shadows. Almost everything of the environment is there to be used and abused - buckets and mops can raise an alarm if you trip over them, closets and chests are there to hide the bodies, plates and grog sacks are there to be poisoned, every part of the interior and exterior of every level is part of the gameplay. The game is also an NPC haven. If you enjoy taking out baddies - each level has them in the hundreds. You can do a quick passive run without being seen, but you can also spend hours methodically taking them out one by one... also without being seen.
It is incredibly fun! As a stealth fan I got nothing but love for this game!
