There are various changes that I think are pretty damn brilliant, a few that were pretty stupid, and some that were a mixed bag.
Great Changes
-Meteor Shower
Fantastic idea. Gives a great reason for Clark's landing without being noticed, and good story telling for the first season.
-Powers Developing in line with Puberty
I like seeing his powers slowly develop, especially the heat vision one being connected with sexual urges.
-Chloe
Most of it has to do with the charm of Allison Mack, but she's turned out to be a great addition.
-Bizarro
By far the best change Smallville ever did. I've always hated the Bizarro character in comics, but Smallville found a fantastic way to update the character. I honestly think DC should change the comic character to this version.
Terrible Changes
-Clark's powers being something that can be taken away
I don't like that Clark's powers can be transferred or taken away, and taken away so easily. Clark's powers come from his DNA and the idea that it can be transferred by a bolt of lightning and some kryptonite, or that some witch can disenchant them, or that Jor-El can take them away at the drop of a hat is kind of ridiculous.
-Jor-El
Has there been any character as nearly inconsistent as Jor-El? Lionel come close, but the writers just change him anytime they need it to fit the plot. One moment he's evil planning a destiny of Clark ruling the world, next its a destiny for Clark to save it, then he enslaves an innocent girl to get Clark, then he's a pacifist who wants to save the planet, then he encases Clark in a block of ice......and I don't have any god damn idea why?
-Flying
Out of all the sins that Smallville has committed, this one has pissed me off to no end. Clark finally flying should have been an amazing discovery of a power, and should have been a gob smacking moment to all in the show. Instead, Clark is like the last kid in second grade that learns how to read. When Chloe told Clark at the beginning of Season 7 that he really needed to get on that, my heart sank. Everyone is just waiting for Clark to fly instead of Clark finding out he can and everyone saying "Holy ****, Clark can fly???"
Mixed Bag
-Lionel Luthor
Lionel Luthor is a brilliant addition to Superman mythology played wonderfully by John Glover, but executed very poorly. Every season, the writers would write him a certain way to fit whatever plot they had that season, and his character is all over the map. He's evil, he repents, he goes back to evil, he's kind of evil and kind of good, he's a protector of the guardian etc.. The writers should have taken much better care of this character.
-Meteor Freaks
Great idea for providing villains, until the writers drove it into the ground. Instead of maybe having a few reoccuring meteor freaks, they decided to give almost the majority of the town meteor freak status. When they got to the T-1000 meteor freak, it just got embarrassing.
-Clark's arrival a sort of prophecy
This works on and off. I don't care for Earth being Kryptonian's vacation spot, but I like the idea that the Kent's were specifically chosen for Clark's upbringing. I kind of liked the cave prophecy, but the whole crystal to the fortress and Veritas thing just didn't work for me at all. It made Krypton's destruction seem so much of a sure thing, that I don't understand how kryptonians thought Jor-El was crazy.
Jor-El knew the planet was going to explode (hence shooting him Kal-EL off the planet) it's not like he didnt have the tech to do something like that.
