Unlike other genres, stealth games are basically about avoiding alerting enemies. They have been one of my passions since the first time I played Metal Gear on my family’s MSX (around 1990, I think).
Technological and game design advances have brought more variety and depth to the genre, adding lights/shadows, lots of tools and greatly improved artificial intelligence. For example, in 2005 Metal Gear Solid 3: Snake Eater (one of my favorites) incorporated a camouflage system to the franchise, allowing players to adapt their looks to their surroundings.
Assassin’s Creed II, in turn, let players blend into a group of citizens in order to avoid soldiers.
In 2012, the genre had a solid 2D implementation through Mark of the Ninja, that chose to represent sound alerts visually, emphasizing the need to act quietly.
Along the years there have been other remarkable examples, of course, like the Splinter Cell or Deus Ex series, although each game has its own interpretation of the genre.
Probably that’s the reason why the guys behind Sneaky Bastards (a blog about stealth gaming) have tried to go to the core of stealth, writing a manifest “asking whether or not the stealth genre as a standalone concept can even be identified.”. According to their essay, the good things a stealth game should are:
They have an interesting take on the subject, including some interviews and a pretty comprehensive analysis. You can find the full manifesto here.