I can tell already that it's going to be difficult to avoid comparing this game to Harvest Moon. I'm about a week into the first season and so far it is familiar in almost every detail. There are a couple of exceptions - crops are not necessarily planted in a 3x3 square (though I initially did so out of habit) and when I went down into the mines to find minerals, there were monsters all over the place.
I'm not yet sure how I feel about this second development. Rune Factory did something similar and I like that game quite a bit, but its controls were a bit smoother than Stardew Valley's and you could use your brush to tame the Rune Factory dungeon monsters and put hem in your barn, which I always thought was pretty cool. My verdict is going to depend on whether the monster loot system is well integrated into the game's general crafting system. If the items you can craft via the various body parts you hack off of monsters can help you in farming, fishing, and ranching then I'll probably like it just fine. If the only thing monster parts can do for you is get you extra weapons and armor, then I'll probably view it as a superfluous embellishment of an already winning formula.
Mostly, Stardew Valley feels like I'm playing a hypothetical sequel to the original SNES Harvest Moon, one which made modest improvements to the graphical style, but eschewed 3D in favor of a larger world and more customization (the ability to move your furniture is a huge improvement over the base Harvest Moon formula).
It's kind of a weird place for me to be mentally. I keep forgetting that Stardew Valley is technically an imitator. And I can't really say why it's important to remember. I mean, this is how genres get started, right? A small game takes some risk to innovate in a direction that hasn't been tried before and is so well-executed that it becomes popular out of proportion with its pedigree. This popularity inspires others to try and improve upon it, with varying degrees of success. Eventually, the imitation games become so numerous that they start to influence each other, and then it no longer becomes possible to cleanly say "this game is a knock-off of X" and you're reduced to saying "games of this type show a clear descent from X" and then BAM, you have a genre. It happened with Donkey Kong and Grand Theft Auto 3. It's in the middle of happening with Minecraft. And it might just be possible that Stardew Valley is the first step in making it happen with Harvest Moon.
Or maybe it's just a one-off homage. I would think that if there were going to be a boom in "character-driven, rpg-style farming sims" that it would have already happened by now. The original Harvest Moon was released in 1996, and while it's had a ton of sequels, I should think that if other companies were going to try and change it into a genre, they would have done so by now.
I think my goal in the next few hours is going to be to try and find Stardew Valley's unique voice. To discover what makes it a worthwhile game on its own terms. It really isn't fair of me to keep comparing it to Harvest Moon, even if the parallels are obvious.