They were in the series, but not in the pilot.
I have to second Ted Danson on CSI. I thought with the departure of Grissom that CSI would deflate, but thought Fishburne brought an interest to the show.
Initially, I thought Danson would hurt the show, but he has been excellent.
A departure that I thought helped was Kyra Sedgwick leaving and Major Crimes really improved.
Then, my Bones McCoy should stand.
On Gunsmoke there were a couple…Festus (Ken Curtis), of course. A one time thing that just worked and worked and worked. The other was Quint (Burt Reynolds). His character fit in well, although I understand he didn’t always.
On It Takes a Thief they had Fred Astaire do a cameo as Robert Wagner’s father and once again, it worked wonderfully and was kept as pretty much a regular. The irony here is that Wagner’s most recent roll is playing a father character much like Astaire’s, but his is on NCIS.
Does not actually qualify, but on Happy Days…No not Scott Baio (or however you spell his name), but Mork from Ork. It added so much he got his own show.
A bad add was Mork’s son. The epicness that could’ve been! Totally misused.
Not quite:
*Four of the original seven pilot characters were identical to those of the series (including the actor/actress cast for each role): Gilligan, the Skipper, and the two Howells.
The three remaining original pilot characters differed from those of the series (including the actor/actress cast): In the pilot, the Professor was instead a high school teacher played by John Gabriel; Ginger the movie star was instead Ginger, a practical secretary with red hair, played by Kit Smythe; and Mary Ann the Kansas farm girl was instead Bunny, a stereotypically cheerful “dumb blonde” secretary, played by Nancy McCarthy.
*
I don’t think Goober added much to the Andy Griffith Show. They just needed a good hearted countrified idiot to replace Gomer Pyle after Gomer joined the Marines. And the cop who replaced Barney added nothing.
Zane was a useful addition to Eureka. I guess Nathan was also.
I went from “oh, is this show still on?” with Fishburn at the helm to looking forward to it every week with the addition of Ted Danson and Elisabeth Harnois. I like Elisabeth Shue’s even more recent character too. I suppose it helps that I’ve always been fonder of Greg, Sara, and Nick than I ever was Gil or Catherine though.
As for Angel and Buffy, Spike and Fred improved both a great deal.
Wait, did this thread really manage to get this far without anyone mentioning Law and Order? A quick look at the Wikipedia article indicates the show went through 27 different main cast members. The two most iconic characters, Lennie Briscoe and Jack McCoy, didn’t join the show until seasons 3 and 5, respectively.
I LOVE Elisabeth Harnois! But Shue’s character has never grown on me. She started off good (hey we need a blood expert, I know a girl) but now it seems like either every case has SOMETHING to do with blood, or she does what ever CSI does and isn’t special anymore.
I heard that this season is going to involve her more, so maybe she’ll grow on me.
Damn you! I was going to say that! 
Have you tried gluing the joints together in a realistic pose?
ETA… user/post combo… 
Kryten didn’t properly join the crew of Red Dwarf until series 3.
The later addition of Kochanski as a regular wasn’t nearly so successful.
I’m a Doper, not an anthropologist.
I have this theory, that Sara killed Grissom not long after they were married, because deep down she’s bat shit insane.
I hope that someday the show writers realize it, too. That would make an awesome plot development.
Hey, if they can make Detective Vega a murdering, cooked cop, my theory could be possible.
Tom Zarek (though he was introduced in season 1) was one of the first real new characters of Battlestar Galactica, and his character development allowed the writers to take some of the card-carrying villain stuff away from Baltar and have more fun with his character, which was one of the true highlights of the reboot.
Gotta respect the season-two introduction of Dean Stockwell as Cavill.
Oh, hell, yeah.
Michael Emerson as Ben Linus in Lost.
“You guys got any milk?” turned it from a show that I enjoyed to a Show I Did Not Miss.
Few days late, I know, but Tywin Lannister most definitely WAS in Season 1. He appeared in Episode 7 of the 1st Season, and then also in Eps 8, 9, and 10.
I believe he was first met in his tent as he was planning to fight Robb, as Tyrion showed up with his Mountain Clans.