Of perhaps the most ironic kind. The producers of Smallville wanted the character of Bruce Wayne to appear. The owner of the current Batman film franchise refused. The irony is DC comics owns both characters and in the comic books both Superman and Batman crossover all of the time, and are, in fact, best friends. Batman finally appears in the Season 11 comics' second story arc.
Stephanie Brown was to appear as Batman's sidekick, Nightwing. But, of course, anyone familiar with DC knows that the higher ups hate her with a passion, so Barbara Gordon was used instead.
Lana: In the earliest seasons she was initially known as the Pink Princess because of her early wardrobe (and the fact that by season 3 the pinkness was arguably becoming a Running Gag until her wardrobe changed in Season 4). Super Kryptonite Woman or The Amazing Kryptonite Woman, after becoming walking kryptonite. Also known as the "squirrel".
Pete: Product Placement Pete, as his little bit of screentime was usually used to place products such as music from Lifehouse, the "Talon Mix" CD, and Stride Gum.
Lois: EDLois (short for Erica Durance), Nois ("No is", as in, "is not Lois").
The episode "Doomsday" is often nicknamed "Failsday" for its rather anticlimatic ending.
Friday Night Death Slot: Was moved to this in its second-to-last season, but remained one of The CW's top shows. Its second season in this slot was its last, but that was less due to time slot and more due to factors such as various contracts expiring and a general feeling that it was time to finally end it.
Michael Shanks as Carter Hall/Hawkman. Oddly enough, this means that that he's once again playing a character with an interest in Ancient Egypt. Also, both characters have also experienced a number of deaths that didn't quite stick. Plus, both characters have similar sounding names for the woman they love (Sha're/Shayera).
Minor examples, but Jodi is Giselle. And a few people may recognize voice actor Andrew Francis as the student who tried to run Chloe off the road for exposing his mother's old life (and that she caused people to die) in Season 3's "Truth." This show gets quite a bit of this, probably because... well, it's Superman we're talking about.
Cassidy Freeman, who plays Tess Mercer, is the sister of voice actor Crispin Freeman (who has coincidentally happened to voice Superman)
And as a possible Casting Gag, Tess was shown to be someone against Clark and working for Lex Luthor before siding with Clark. note Crispin voiced Superman in a JL game and was the voice for Red Arrow in Young Justice, therefore both Freemans have been allied with the Big Blue Boy Scout.
What Could Have Been: Allison Mack toyed with the idea of auditioning for Lana Lang before going for Chloe Sullivan. Incidentally, Allison is a green-eyed Caucasian just like traditional Lana while Kristen Kreuk/Smallville Lana is part-Chinese with dark eyes. While Allison is a natural blonde, the actress of Maxima also dyed her hair from gold to red for the role.