Here is a pretty full list of the changes they made:

http://en.thewitcher.com/patch2/

I like a lot of the changes (though I am no where near playing in Dark Mode myself! :P), except that they changed the Parry mechanic and made it worse in mine and some other people's opinions which is unfortunate. Geralt can now always parry regardless of vigor (good), but never does it at all well - gets 50% of the damage each time (very, very bad).

If you played at launch (ver. 1.0), the first thing you gonna notice is the combat.
Is more fluent ... and easier. I'm not very skilled, so is a good thing to me.
I'm playing on hard, building an alchemist Geralt, and beat Letho at the elven baths at 3rd try. So ... is not that hard.

But if you are skilled and you are looking for challenge, you may have find that challenge at the arena. Or you can play in dark mode and if still is no enough challenge, add a mod.

There are a lot of little changes in all the patches from 1.1 to 2.0 :
* 9 DLC
* the storage
* F9=quick load
* delete saved-games from the "load game" screen pressing delete key.
* "bulk alchemy", i mean, If you want 10 cat potions, you can make the 10 potions in 1 operation, not one by one.
* obviously: dark mode, tutorial and arena.
* etc ...