One of my favorite 'genres' of play, there's quite a lot there that doesn't get nearly enough credit for what they can bring to the table. There's a ton of variety in both the stories they tell and the way they approach them, and the creators are often allowed to invest significantly more time into making more interesting worlds that would otherwise just be backgrounds. I think if one is able to look past the negativity surrounding the term, there's a little something for everyone:
Dear Esther
The story of a griefstricken man mourning the loss of his beloved Esther. Uses unreliable narration and fragmented storytelling to key the player in on the events that occurred. One of my favorite features is that the narrator has several "moods", between sorrowful and outright hallucinating, and the game randomly decides how his mood will swing as you progress. Some find his victorian purple prose to be too angsty or flowery for their taste, but it's quite fitting and refreshing as far as video game writing goes.
Proteus
An audio/visual toy with no dialog or plot to speak of. Yet, there are telltale signs of some form of life that was left behind on the island, and creatures still inhabiting whom are going about their business without the interruption of people. You're left to explore each island as you desire, moving seasons until the final moments of winter. It's a serene experience that demands nothing of you, which some don't find to their liking. But if you can let go for a while, repeat trips can be quite relaxing.
Eidolon
Society has crumbled in the upper northwestern United States, and you a survivor must dig through the rubble of cities toppled by a mysterious event and the surrounding woodlands which has slowly grown over the old signs of mankind's concrete blasphemy on Earth. The story is told through diaries, text logs, blogs, newspaper clippings, and more artifacts as you follow several characters in their final months/years before the big event occurs. Features some light survival elements, though it is fairly unobtrusive compared to other survival sims.
Ultraworld
This is one of the few even I'm not completely sold on. The locales are fairly pretty and the music can be catchy, but the overarching plot relies quite heavily on your ability to empathize with the AI who controls the world's dilemma. The abrupt ending and subsequent breakdown will either make you feel compelled to follow the AI's instructions, or laugh your way all the way to the uninstall button. I followed the AI's instructions, but I'm disappointed in how little a difference it seemed to make.
NaissanceE
One of the most oppressive and mysterious games I've played, NaissanceE is akin to walking through an Italian art film about ever-expanding Brutalist architecture. Despite the minimalist approach, the environmental design speaks volumes about what once may have been this strange places' purpose, while still never giving you any direct answers. The choice of music, the path of progression, and the gorgeous areas make this stand above many others in the "walking sim" space and, in my opinion, is one that everybody should be looking at.
FRACT OSC
FRACT OSC isn't quite as loose on the gameplay elements as other "Walking sims", and in fact, is quite heavily integrated with its musical puzzles. I stick it here though as it does have a lot in common with others in the genre. The main goal is to reactivate a giant musical computer which happens to be ingrained in the surrounding caverns. Your reward is twofold - both a more lively game world, and special features unlocked for the game's built-in Digital Audio Workstation. In a way, it's trying to teach you the basics of how to construct a dance song. A must for those who grew up watching things like Fantasia as a kid and somehow wound up with a stack of Autechre albums as an adult.
Gone Home
Gone Home takes the audio/data log trope of every game that's come out in the past 10+ years and applies it to an intimate, personal space. You're a girl who has returned home from college to find her family not there when she gets back. No biggie, right? At least until she finds evidence that the empty home isn't just because everyone felt like grabbing a bite to eat before you rolled in. How much one can appreciate Gone Home will depend on how heavy their nostalgia is for the year 1995 and if they can find themselves invested enough in the interwoven drama of the family. Sometimes, the best stories are the ones about ourselves.
Jazzpunk
Jazzpunk is another one that requires a bit more direct player involvement than most, but it all really leads to taking in the hundreds of gags in your surroundings. Jokes poking fun at modern gaming with a dose cheesy 60s spy flicks will overload your senses with their rapid-fire delivery. This one leans heavily on if your sense of humor is compatible with the game's jokes. If you enjoy anything that Mel Brooks had directed, or enjoy the finer points of the movie Airplane!, then Jazzpunk will be your guide to madness. On a related note...
Thirty Flights of Loving
Thirty Flights of Loving is an absolutely incredible experiment in telling the most amount of information as possible in the least amount of time. Your interaction with the world varies from scene to scene, but the way it compresses so much information about the world into your surroundings and actions of your supporting cast is one of the shining moments for "Walking sims" as a storytelling medium. At first I didn't understand it, but shortly after I came to look at it from a different perspective. Yes, the game's pretty much over in 15-20 minutes, but it's so crammed that you'll find yourself thinking about something the game showed you a week later and only then come to realize just how important said thing was, and you never realized it at the time.
The Moon Sliver
It's not the prettiest game around, yet I found myself almost completely forgetting about how basic the environments were thanks to the well-executed sound design and the power of the story. You awake on an island to find all the inhabitants no longer around, leaving you to discover where everyone went. The gradual introduction of elements of both human and supernatural drama make every discovery even more affecting. It must be said that for as short as the game is, it instilled a sense of dread in me moreso than many other modern horror titles.
The Vanishing of Ethan Carter
The detective life has been tough on you, but you just can't resist this one last case... a letter from a boy named Ethan Carter has drawn you to the islands where he grew up and eventually disappeared. The game's world is very realistic thanks to the devs' use of a special technique applying real world photography into a 3D space, but it still manages to feel larger than life. Your discoveries will make you question just what happened, comparing the results of his and others' actions to the tall-tales Ethan spins about them.
The Stanley Parable
An absolute must for anyone on GAF, or any gaming enthusiast site, The Stanley Parable is a no-holds-barred approach to lampooning current gaming trends and tropes we often take for granted. There is no fourth wall here. Hell, I don't know if there were even three before that one to begin with. It's a complete deconstruction of the industry, distilled into one game with a ton of options and tricks up its sleeve. And yet, the best part of it all is how it slowly unravels and lets you discover the different endings. Some of them are very obvious, with obvious Point A to Point B progression until the end, but others require thinking about what you're doing... and then doing the exact opposite. Or maybe just tilting it on its side. Get creative, Stanley! Also worth playing the demo, which is 100% original content from the original game.
...Alright, I think I've covered plenty! But like many other genres, the games vary pretty wildly from one another. So, I think it's worth exploring each one on your own instead of trying to view them as a whole. To those of us who love them all, the differences between them are as significant in the "Walking sim" genre as the differences between Counter-Strike and Call of Duty: Modern Warfare 2 in the "Shooter" genre. Dig in. Dig deep.