I like games with a big open world. There are tons of interesting and diverse side missions. The main story is awesome. And the variety in dialogues make me feel like every game is different. Depending on questioning and answering, you make your own story. There are a lot of cutscenes, some pretty long even. So, you really feel like it's a movie sometimes.
The game is dark, sometimes even depressing. I didn't feel much for it at first, but you will get into it thanks to the storyline and characters. It requires some patience. Personally, I enjoy weapons like swords and crossbows in games over guns. Alchemy and crafting is really diverse too. I have no sight on the amount of potions, bombs, etc and equipment and weapons you can make, but from what I see...endless possibilities. The Witcher can cast spells to help you in fights or get through obstacles in the open world.
Safe to say that each gamer has its own kind of Witcher, as you can upgrade skills and abilities in your Witcher's personality. You get a point each time you level up. Levelling up differs, experience points are easier to gather when you play on an easier difficulty level. You can improve in general fighting, alchemy and spells. Once upgraded a skill, you can assign it to a slot. There aren't unlimited slots so you cant be best in everything.
Also, there is a game of cards called Gwent. Pretty addicting as one of the side missions is to collect all cards. You can play innkeepers, landlords, merchants and other excellent Gwent players listed in the side quest. Imagine it like Pokemon and Yu-Gi-Oh with more factions.