Berke Breathed acknowledged that Bloom County didn't really take off until Opus, Binkley and Milo became the central focus. He admitted that early on, he didn't know what direction to take the strip (leading to a massive Retcon of nearly the entire first year and a half), and that he often cribbed Doonsebury in the earliest strips.
Calvin and Hobbes opened up emotionally with the baby raccoon arc. It opened up creatively with the transmogifier arc.
The "big dog that stole Hobbes" arc predated the baby raccoon arc, and was the first thing to really open sentimentality into the strip, setting the stage for baby raccoon. It also let Watterson know that the readers were connecting with the characters as he began receiving letters from fans who were worried about Hobbes before the story even resolved.
Dick Tracy has suddenly come back to life with the retirement of Dick Locher and the hiring of a new team.
Doonesbury was originally about a group of kids in college for the first decade or so of its run. Gary Trudeau took a 2-year hiatus and then began drawing the strip again, developing the art style and real-time storyline that the strip is known for today.
While the art definitely improved after his hiatus, the writing of the strip had always been top notch. See Trudeau's Pulitzer he won in 1975 as evidence of that.
For the first few years, FoxTrot was just a funnier, quirkier version of the typical family gag strip with several of the cliches (Older brother's eating habits, daughter's ditzy behavior, father's cluelessness, etc.) turned Up to Eleven. And then, in 1989, Jason dressed up as Batman to go see the new film, introducing the film's pop culture awareness and geek sensibility that would eventually come to dominate the strip.
For its first month and a half, Garfield was centered on the titular cat and Jon. The introductions of Lyman and Odie made the strip one of the most familiar to many readers.
Peanuts: in the first few years after its 1950 debut, it was a basic gag strip about children, with a few odd quirks (a kid playing Beethoven on a toy piano, a smart dog, etc.). Then in a 1956 sequence, Charlie Brown got his kite caught in a tree, and was so angry he decided to just stay there. This went on for over a week, with other characters walking by and making sarcastic or inane comments. Charles M. Schulz himself later identified this sequence as the moment when the strip's unique brand of humor finally took shape.
Another often-cited (by critics and creator alike) example of a turning point in Peanuts is Snoopy's Anthropomorphic Shift: gradual for the most part, but one day he began walking on his hind legs, and there was no turning back.
The Perishers initially featured strong artwork and occasionally funny jokes, but was marred by generic characters and an over-reliance on Running Gags and cutesy moments. This changed when initial writer Ben Witham was dismissed and replaced by Maurice Dodd, who fine-tuned the strip's humour and made it ten times better in the process.
Sally Forth (Howard) started out as a prosaic 'liberated wife/mother in the workplace' strip, until Francesco Marciuliano was hired as writer, putting a cheerfully goofy-yet-grounded spin on things.