It's bascially like a Harvest Moon game but further expanded upon with RPG elements

The thing that sets it apart is that you are truely free to do what you want. The issue I have with Story of Seasons is that everything is too slow at the beginning. My main issue with that game is that there are too many tutorials, making most of Spring a tutorial season. You can't sell items every day and you burn through your energy really fast. In Stardew, you get access to the mines on like Day 3 or 4, allowing you to collect minerals to upgrade your weapons right away. Mining is nice, as there are nodes with guaranteed minerals and your floor progress is saved every 5 floors.

The crafting system is a great addition, allowing you to make things whenever and wherever you want, provided you have the materials for them. You can also craft items that can help water your plants or scare away crows, making farming easier. Newer Story of Seasons games allow you to place buildings freely, but the amount of freedom is expanded upon in Stardew. There are goals to do in Stardew, giving you rewards when you complete them, and can further help in developing your farm.

Harvest Moon did take a step to making farming more complicated with having enough sun and enough water, but too much water can kill the plants. I prefer it being simple, but I find that Rune Factory really found the sweetspot with simplicity but having enough factors to make farming more in depth.

The combat isn't the best but it's a great addition by adding another element. Rune Factory's combat system is much more developed, with a variety of weapons and skills, having party members, taming monsters and level up.

Stardew is basically Rune Factory, but more Harvest Moon based :p