Well, I've only played the game once so far on Hard difficulty and I have to say, it wasn't as bad as most people describe. I'm a seasoned player of the first game, but past experience I've noticed is of little bearing - everything is just so different. That's where my main problem was - the prologue saw me approaching situations the Witcher way, when all the combat system had become much more dynamic and "twitch-based". That's where I died the most. Chapter I was difficult in the beginning as well, especially the Nekkers in the cave and the group of Scoia'tael that ambush you around the lake if you make the appropriate choice in a quest. And the Letho fight was just a nightmare. But a nightmare that, ultimately, taught me how to play.
See, I think the biggest problem for most people with that fight is that he uses his entire arsenal against you - bombs, Signs, fast and strong styles; in short, everything which was pretty much exclusive to you up until that point. There's just nothing that can prepare you for Letho suddenly breaking your block with an Aard sign, for example. (something which caused me to restart the fight more than once) However, it also means that by passing that battle you're pretty much prepared for whatever the Wticher 2 may throw your way. And, true enough, I encountered little difficulty after that fight (setting aside the occasional screw-up on my part). I found out that I now had a new-found respect for dodging and the Quen sign which I had almost never used in the first game!
It was a drastic change in my playstyle - throughout the first Witcher, I had used mainly swordplay, Aard and the ever-convenient Swallow. In the Witcher 2 I used a lot of dodging and measured, tactical blows. I think the experience is actually quite different - it gives you the impression that you are this extremely agile fighter, but you have to play to your strengths.
To put it simply, after I got the grip of the new combat system, everything else was a breeze. The Kayran fight was tricky, you had to have good timing there, but the most annoying part of it was the QTE which caused me to restart a few times. I just couldn't make out what button I had to press, weird I know - considering I never lost a fist-fight in the game.
In fact, due to the nature of the new potion system I passed most of the fights without drinking anything. I just didn't know when to expect it. Or once I actually realised I had to, the area would not allow me to meditate. As a result, I passed most of the (boss) fights with only my trusty sword to see me through. The Draug and the Dragon fights in particular, I kept feeling that they would be much easier if I had a Swallow beforehand, but ah well...a few restarts down the line, I managed to see them through
All in all, I'd say that Hard was not particularly hard. I had a myriad of potions and bombs by the end of the game which I hadn't even bothered to use once. Something which troubled me during the first game as well. I now believe that I've missed the point of those and some situations would simply have been...different to say the least, if I had bothered to drink them.