There has been a lot of talk going on about this game in the last weeks and this is one of this "surprisingly well praised releases" that suddenly popped up and was immediately liked.
Basically the game is a spiritual successor to the Harvest Moon series. Stardew Valley takes most features of those games but leaves out some of the more annoying aspects. it adds also some new ideas to the mix in order to differentiate it from Harvest Moon.
People who know already the HM series will know, that Stardew valley is a farming/life simulation with some slight Dating Sim aspects. You play as a guy (or girl in this game) that acquires a farm in a small town but when you see the farm you realize it is currently a mess, most stuff is broken down, the farming plot is overgrown with weed and treestumps, and you have to pick up the rocks, that are littered across it.
You literally have nothing to begin with, only a small amount of cash, some basic tools and some seeds.
The goal is to renovate the farm, earn some money and spend the money on improvements for the farm. You remove most of the trash on the plot and start sow some seeds. Eventually you will harvest some crops and put them into a chest, which gets emptied every night. You get paid for the value of crops, it works pretty much like the tooth fairy. The earlier games (and I think some of the later) of Harvest Moon had a time limit, where you had only x years until you had to improve the farm and earn enough money.
Besides crops, you could also, after building stables, buy soem animals and raise them, adding a unlimited source of products.
Between your farming life, you could go to the near town and talk with some of the people there, earning their friendship. You could aslo give some presents to female characters and eventually marry one of them if the like you enough. After marriage you may get children. In most of these games there is an end game goal but you could still play on if you wanted to.
Stardew valley takes all that and blends it with some own ideas. What I like is, there is no time limit but this is given, newer Harvest Moon games had discarded the idea of a time limit anyway because player didn't like to feel rushed. What i prefer on Stardew valley is the crop system. In Havrest moon you buy a full package of seeds and your character just tosses them in the air, sprinkling them on all adjacent plowed fields. This led to the fact, that everyone designed their fields like this:
There was also the annoying fact, that crops, when grown to a ertain point, became unpassable, so unless you have upgraded your water can to reach the center field, you had to leave a field open in order to move to the center (or just ignore the center to begin with).
SV instead lets you place seeds on a single field. That means that you have to do more work but on the plus side, the packages are alot cheaper and you can place them like you want. And to make it better, most crops don't block your movement unless the crop needs a wooden support (like beans). This gives you a better control how to arrange your fields.
Similiar to Hm you can upgarde your house and build some stables for animals. However, this time you don't rely 100% on town residents. The character can do some basic crafting on his own, for example you can at some point craft sprinklers that automatically water your crops. bigger procets still require to employ carpenters or blacksmiths.
What me surprised the most is, that there is combat in the game, though in a very basic way. You can explore a mine in order to get some minerals, however the mine is infested with monsters. They attack you and you can kill them with a sword. You can even tame the slimes for your own farm. I don't think this addition to the game was needed, however I remmber farming the mine for metal was always the most boring thing in HM, so maybe the developer wanted to live it up a little. It this was a success has the individual player to decide.
Your character gets also experience for different skills that you practice while doing your job. Foraging, farming, combat, fishing and some other stuff. If you level the skill up, you can craft more stuff related to said skill and you get a little better in it. it is nice small addition to keep the player going.
Considering this game was made by only one person, it has a very depth gameplay and is worth to check out for fans of Harvest Moon.