Originally Posted by kalniel
TW1 got away with it because of the amnesia thing, TW2 tells the story of Geralt getting his memory back, but you're left in the uncomfortable position of Geralt starting to remember about key characters (Yennifer, Ciri etc.) while it's the first time as a player you will have heard about them. Several in-game consequences are then dependant on these previous events (as described in the books), such as Triss' reactions and decisions, but if you haven't read the books then these seem to come out of nowhere.
The background required isn't at all addressed in TW1. The prologue of TW1 addresses the pertinent points to the gameplay of TW2, ie, Geralt's memory loss and what Witchers are etc. but has next to nothing about the characters important in Geralt's life that are relevant for the main story reveals in TW2.