I have really been enjoying The Witcher 2 (which I finally got around to picking up when the 2.0 version was released).

What are the parts of this first one which will be hard to swallow after enjoying the second adventure?

A second question: should I consider pausing my playthrough of The Witcher 2 (I'm halfway through the second chapter) and start on this one instead?

Don't pause your TW2 playthrough.
Game mecanics are diferent and the switch is enoying.
Give TW2 one ore two full playthroughs and the move to TW1.

TW1 is a longer game. And is a more classic rpg. I think TW2 is an action RPG insted.
And also think you may have to learn to tolerate is the load times whe you travel between locations.
The faces of the characters has changed.

My advice is to search the game in youtube a see it by your self.
This will give you pretty good idea:
http://www.youtube.com/watch?v=7fL77c6nTc4

Thanks, jjavier

You'll probably notice the mechanics first. Combat in TW1 is timing-based, but if you play in Easy or Normal, there's an icon that helps you learn how to do it. It's less "involved" than TW2, but the combos still look nice. Also, read books. You can't do proper alchemy or even gather most ingredients without them. Sell them after reading them b/c you keep the knowledge in your journal, unlike the diagrams of TW2.

Finish TW2 first. Go back to TW1 after that for more development of the world and its characters. Forgive it for being older and having less experience and fewer resources behind it. Appreciate it for being awesome in spite of that.

I just bought The Witcher Enhanced Edition Director's Cut from GOG last night and played the intro bit. I've been playing Witcher 2 all week and the difference was really jarring at first. But I really enjoyed what I played of Witcher last night. I think I'll finish Witcher 2 first though since it's (imo) a better game. I'm more of an action RPG fan and the combat is right up my alley. The combat in Witcher is different and timing based (as stated), but it's still fun. I'm kind of in the same boat with Deus Ex. I played Deus EX Human Revolution, but never the first one that everyone raves about. It looks so OLD now. I'm gonna try it though.