It's a beautiful game. Hard to believe it shares an engine with NWN (which it practically doesn't... I think they said that at the end of it, they rewrote like 90% of the engine and really just had the core). But yes, the story, characters, world, and major decision-making bits are the most important elements. The other stuff is good, but don't expect too much customization. You'll help Geralt make a lot of significant choices, but the color of his boxers won't be one of them.
Oh, yes -- a lot of decisions you make, you'll see the importance of your decisions usually sometime later on; maybe one chapter or two chapters down the line. You won't see it sometimes as soon as you make the decision, but later on, you might.
This isn't a OMGPHATLEWT game by any stretch of the imagination. There's very little in the way of gear, though apparently there is some. And yes, every change you make is reflected on the character (at least that I've seen so far). But you're playing Geralt because... the main character is Geralt. This isn't a build-your-own-Witcher type deal. The only customization you're going to be enjoying is mechanical, in terms of what abilities you choose to increase (and even that doesn't change the way the game plays all that drastically, just which skills you'll likely favor).
About tricking out your character, basically at level-up time when you meditate, you just decide if you want to learn a few new skills AND often it might be on how much more powerful you want to make a certain Witcher skill. There's around 200+ skills to learn.
You can upgrade your weapons and armor. Maybe even say buy a few new weapons and armor, but there's not a lot of new junk to get. There are a few quests, side and main, in which you can get new items or are given new item. Basically, you'll do more upgrading of your current weapons and armor, more so than anything -- if you can actually afford it. By doing lots of quests (such as side quests and Witcher contract quests), you should be able to do a fair amount of upgrading.
Plus, you can't basically carry many weapons w/ you, as is. Just a Steel Sword (for humans), Silver Sword (for monsters), and TWO extra weapons (a dagger, torch, axe, etc etc).
You can carry lots of potions, books, food, plants, and other smaller stuff, though.
You should spend a lot of time mixing potions -- especially w/ Recipes you might find, so you don't waste the plentiful amounts of raw materials. If you're having trouble w/ a battle, usually brewing some potions AND then using them potions in battle will be just enuff to get you a victory.
