ETA: not going to bother typing it out again but:
http://www.gog.com/en/forum/the_witcher_2/bad_reviews_what_do_you_guys_make_of_it/post46 Basically, you shouldn't be thinking of the chapters as 'amount of stuff to do' but how it advances the story, and the feeling it conveys, since the story is absolutely the most important aspect of the game.
revial: The fact that you think Letho was your nemesis speaks volumes of how much you actually took from this excellent story. Also, you haven't really finished until you've played both sides, as each side is only half the story.
Bonaventura: Meh, he was the closest thing to it.
I mean, am I really supposed to care about nonhumans or politics as a Witcher?
As Geralt himself put it once "I'm a Witcher. Neutral as all hell.", I played through TW1 as a neutral character and all of a sudden I'm supposed to care about the schemes of sorceresses or about Nilfgaard?
What about the wild hunt?
Turns out they're probably some kind of extra dimensional elves...what? :v
Letho is the closest thing that can be associated with a rival.
You're the most wanted man of the realm because of him, he kidnaps Triss to make his getaway from Flotsam and is you're told multiple times that he's got some important knowledge about Geralt.
Then at the end..."oh hai, I was a Nilfgaardian agent all along! Also, I don't really give a shit about you, but if you wanna fight, you know, I'm totally up for it! Also let's get shitfaced with vodka!"
I mean come on, what kind of ending is that. :v
I decided to fight him, only because I wanted a boss fight...but by that point I was like "so he's working for Nilfgaard...really? That's IT? pfffrt" I didn't really care. :\
While the Wild Hunt wasn't a huge part of it, it's because it's clearly foreshadowing for 3. In 1, the Hunt was pursuing you but you didn't know why, and never really learned. In 2 you learn more, and regain some of your memory in the process, and in 3, it will probably all get resolved. It's this multilayered storytelling of the series as a whole, but was especially apparent in 2, which is one of it's best aspects. And the elves thing does fit the history of the Witcher's world, the entire history is based on extra-dimensional invaders of all sorts.
And while Geralt puts on a front of neutrality, it's clearly a defense mechanism, he most definitely has opinions on the political/social situation of the world in the books, and at times acts on them. This sometimes backfires on him (this is what got him killed) but othertimes it's the only course of action, and works out allright... ish.
As for Letho, yes, he has many aspects of an enemy, but in the Witcher, nothing is ever black and white. He's not strictly evil, he doesn't hate you, he just has goals counter to yours, which you got caught up in. That's the whole point, the message you were supposed to take away. Whether those goals of his are worth killing him is up to you.
And you should care that Nilfgaard is invading, they are a massively powerful force, and it sets up an interesting scenario for 3, considering Geralt has to go there to find Yennefer. While it definitely has personal aspects, TW2 was a 'big picture' story for the most part. And yeah, if you aren't paying attention to the larger picture, it will feel shallow. But if you do pay attention, it's incredibly rich and complex.