Witcher 3 is different from the first two games in several ways - all of which make it an even better experience to play in my opinion. First of all it is an open world game (albeit some areas are cut off at the very beginning of the game). Arguably the first two games also had open-world elements in each area, but ultimately they felt very linear to me. The game touches upon darker themes than the first two games, but it is not as "noir" as Witcher 1.
People disagree a lot over the combat system. I always compare it to the Souls games (Demon's Souls; Dark Souls I/II/III) because combat evolves around learning the movement patterns of your enemies and being an intelligent fighter rather than just charging in. I very much disliked the combat system in Witcher 2 (the one in Witcher 1 was silly, but somehow worked), but I LOVE the Witcher 3 combat system. On the highest difficulty you rely on bombs, signs, oils and potions (which now work brilliantly), but ultimately win fights through being a thinking fighter. Again I should stretch that many probably disagree with me on this - I am just giving you my two cents. However, I recommend playing the game on the highest difficulty from the start as this is the best way to truly learn the combat system properly and gives the game the right "feel". (Again just in my opinion)
When it comes to folklore and story-telling Witcher 3 is arguably the best (or one of the best) game(s) ever produced. Period. A lot of people dislike other parts of the game, but still praise the story-telling - simply because it works SO well. Most of the storylines are interesting, involve difficult choices with consequences and are well presented. You will accumulate lore about bestiary, characters and locations through playing the game and reading the numerous books that can be found.
I should warn you that I am "biased" in the sense that the Witcher 3 is a pure 10 out of 10 for me (currently on my 3rd full clear of the game. I have started from scratch again every time a new expansion has been released). There are probably people who are more critical than me.
Oh; and THERE ARE NO QUICK TIME EVENTS! :D
I hope you buy the game! I promise you the gaming experience of a lifetime. Having played computer games for more than 25 years I call Witcher 3 the best game ever without flinching.
PS: If you want eh ultimate experience read the books before playing the game. They are brilliant - and all (except one) now exist in official English translations.