Well, Miranda was played quite differently during Season 1 of SEX & THE CITY. I don't know how to explain it, but Cynthia Nixon's portrayal was bitchy but not really as well-rounded as what we ended up seeing towards the last couple episodes of the first season. I feel Samantha was more or less the same though. Carrie was less fabulous at the beginning, unfortunately by the end of the show she had gotten annoying. Anyone else remember that device where all the characters spoke to the camera mimicking Carrie interviewing people for her column? It stopped in Season 2, I believe.
If we're talking all shows and not just sitcoms, Alex Karev on GREY'S ANATOMY was entirely different in the first season (or maybe the first couple of episodes).
Also, the character of Sarah from BROTHERS & SISTERS was a bit more restrained, when watching the Season 1 DVD, you can tell when the character became the Sarah Walker we all came to love.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"