Play it if: you want simple, accessible puzzles and a short, sweet family-friendly game that's big on humor and character interaction.
Don't play it if: you're in the mood for something long, challenging, or particularly serious.
It's difficult to say something particularly new about a game like this. With its small scope and broad appeal, a lot of the obvious things have been already stated. But I'll go ahead and try to unpick what I like about this game anyway.
The appeal of Lost Pig all about its main character. Grunk is ostensibly the narrator of this tale, so his attitudes towards things colors the player's entire experience. I find Grunk's fairly simplistic descriptions of things interesting because they are reflective of the archetypal IF player's experience. Like Grunk, we put ourselves into a situation where we're confronted with machines and mechanisms we don't really understand, and we're made to figure out how to use them for our purposes. Grunk describes the world with the naivete of a child, and more importantly the naivete of a first-time player. And like the player, Grunk overcomes that naivete with cunning and shrewdness. Sure, he's not great with auxiliary verbs, tenses, or writing, but he does figure out all the steps needed to get the pig back. And with the addition of the gnome, Grunk can even display a fairly deep level of curiosity by learning about advanced principles of chemistry.
So Grunk's traits are those most IF players wish to cultivate in themselves: intelligence (in solving puzzles) and curiosity (in talking with the gnome).
Add to this his sense of humor. I've always held a soft spot in my heart for those throwaway pieces of coding, like Zork's patronising response to jumping for no reason ("Wheee!" "Very good. Now you can go to the second grade"). Little bits that added some personality to the game world and gently steered you away from the game's inability to let you do absolutely everything. Lost Pig almost feels like a game that's composed of that stuff. A lot of the joy in the game comes from having Grunk try stupid or outrageous things just to see his responses. And because the game is so thorough in implementing the things Grunk can try or do, it gets you to sympathize with him even more. Grunk lets the player act out the more childlike side of their sense of humor, because his willingness to try anything mirror's the player's willingness to make him do anything. Take the act of Grunk taking off his pants in front of the gnome. By itself, it's not particularly funny. But the fact that we're complicit in that act does make it funny. While playing this game, I found myself laughing at stuff I haven't been able to laugh at since the fourth grade.
That may not sound like a compliment per se, and I suppose it isn't if you're looking for something a little more literary. But I think the point that this game has constructed a uniquely sympathetic and charming main character stands.
So what about the secondary aspects of the game? Well, there are no obvious holes in the implementation of the setting. The gnome and pig are lovely characters in and of themselves - the pig for his variety of emotions and reactions (including intellectual disdain for Grunk!), and the gnome for the breadth (if not depth) of conversation you can achieve with him. I liked immensely the fact that the gnome is not immediately hostile towards Grunk - I mean, Grunk could realistically eat the guy - nor is he dismissive towards this comparatively dim and uneducated protagonist. Rather, he's willing to talk in basic terms about most any topic Grunk can think of and a few more besides that. For that he becomes a likable character and his relationship with Grunk, small in scope as it is, compelling.
The puzzles are few and simple, but they rely on intuition rather than method (intentionally so, as the maze demonstrates) and so they give you the pleasure of experiencing those little "eureka" moments every puzzle designer strives to cultivate in a player.
If I had something I'd change about this game, it would simply be the length. The core magical mechanism feels productive for more diverse and complex puzzles than what Lost Pig gives us, and even putting that aside I would have loved to have seen a game where the rather simple initial quest gets this young orc embroiled in something a lot bigger. Failing that, I think Grunk is easily a rich enough character for future adventures.
But that quibble aside, Lost Pig really is a gloriously fun and engrossing way to spend an hour or two.