You don't need to have played the first Witcher, no. Being familiar with the world certainly helps, but it's not a necessity. You're likely going to be confused regardless, because they throw a bunch of politics at you that you couldn't possibly know anything about, but things become clearer as you progress and pay a modicum of attention.
That said, I liked the first game a lot. The biggest difference to W2 is the combat, which is rhythm based. Click when the cursor indicates, essentially. But even though it can feel detached, you're still required to change stances (depending on what type of enemy you face and how many there are), switch swords (steel for humans, silver for monsters), dodge and maybe cast some magic. You can also buff yourself and your weapons to prepare before major battles (there's a trail of hints leading up to each boss), but it's not really required on normal difficulty. There's a big emphasis on knowing what you're facing and how best to counter it, and although it's not very deep it actually makes you feel like you're the mutant monster hunter/fantasy pest control the game tells you you are.
If I were to criticise anything it's the length. The game kind of drags and the finale is just weird and nihilistic, but you can always stop when you get bored and move on to Witcher 2, which has a more intriguing plot that touches on a lot of the same things the first one did (human VS fey folk race tensions, political scheming, the place of purportedly 'neutral' Witchers in the world, etc.).
