Storyline wise, how much will I be missing out? I've never played the first one, perhaps someday I'll go back and play it, but I'd like to try out this one. If anything, should I just go back and read the plotline of the first game while I buy/download the sequel?
It'll be fine. You might not catch the significance of certain characters and how they relate to Geralt, and there's a few references to what happened in the first game... but it's not a major detail and overall you'll still enjoy the game. It's definitely a nice addition being able to appreciate what you have learned from the first game though. --- This text occupies the signature section of my post.