This thread is not for rants about Smallville if you happen to dislike the show. It's a way for all fans to compare the show to the mythology and tell what you like that they changed, and what you haven't liked. Please keep it civil, people.
I'll start off:
Lionel Luthor - Okay, I wasn't really as huge a Superman fan before Smallville because I was just getting into comics as it was, but from research upon watching Smallville, Lionel was never too big a part of the comics, and he only had a few mentions here or there in the movies, even though he's not directly stated by name.
In Smallville they had one of the best actors of our time as Lionel, which is John Glover, and what we saw from the character was his transition from being evil to trying to redeem himself and show Clark he was a father figure. I always enjoyed watching Lionel, whether he was villain or good guy, because he always brought something interesting to the table. The fact that he died protecting Clark's secret shows he truly changed and redeemed himself, and Smallville in a whole has added a big peice of meat to the character that is Lionel Luthor.
Hopefully he'll be appearing in the comics a lot more from now on.
Bizarro - Something that always bugged me about Bizarro was that he was a villain who could go toe to toe with Superman in a fight, but he wasn't smart. Smallville took the character and made him respectable. Rather than being a clone that goes bad, he's a wraith that borrows Clark's DNA, and while they are still vastly different, they still share similarities, as they always should. I'd like to think the comics will continue this notion rather than having Bizarro be a total & complete idiot, because that version just isn't cutting it.
Meteor Shower - On other message boards, many people agree that it's better that a meteor shower came down along with baby Kal-El's ship, because in this day & age the government would be all over a spaceship landing in a heartbeat, and by a meteor shower occuring at the same time as his ship's arrival, it helped to keep him even more of a secret.
The meteor shower bringing down a crapload of Kryptonite was a great idea, as well. I don't like my heroes to be all powerful, and whenever Clark is close to saving the day, Kryptonite rears it's head, and prevents him. The writers have always had a good balance...not using it too much, but also making sure that it shows up often enough.
Lana Lang - I know I'm going to catch hell for this, but I absolutely love everything that's got to do with Lana as far as Smallville is concerned. People complain that she's annoying & what not, but it's a pretty safe bet that any woman would be the exact same way if they had to endure what she has. She's seen friends of her's die all around her....she's nearly been killed dozens of times...and even worse, she saw her own parents killed in front of her.
The character itself has matured a lot by now, and Kristin Kreuk has become a sensational actress in that she's able to show how well Lana has matured.
Pete Ross - A few people I've run across are mad because of the fact that Pete's black, not because they're racist, but because to my understanding in the comics Pete was always able to cover for Clark if something ever happened, to throw off suspicion. I don't think Smallville needs that, and Sam Jones was fantastic as Pete. It just sucks that he didn't stick around longer than he did.
Doomsday - Doomsday has to be one of the greatest achievements of the show. Aside from "The Death of Superman" arc, just about everything Doomsday has been in has been utter crap. Smallville has taken him & made him more than just a Space Hulk. They've given him depth, and I particularly like that there's a man inside the monster, but the man is just a cover up for what truly lies beneath, and the man inside is slowly fading away.
Getting Sam Witwer for the part was a great choice, because he's a great up & coming actor, and hopefully Smallville gets him the recognition he deserves. In any case, he's great at portraying the character as having a darkness within, and knowing that something horrible is inside him, but he's unwilling to accept it.
This is all I'm going to put up for right now, but there's tons more.