This is what I got; can you guys think of anything else to be considered the "eras"?

The Richie and Potsie era (the original 1971 pilot + seasons 1-2)
The Fonzie Era (seasons 3-5)
The Early Joanie and Chachi era (seasons 6-7)
The Joanie and Chachi era (seasons 8-11)

Happy Days could be broken down into three eras.

1. The one camera era that focused on Richie, Potsie and Ralph.
2. The live audience era where the focus shifted to the relationship between Richie and Fonzie.
3. The post Ron Howard era when he left to be a full time director. Joanie and Chachi's roles expanded and Roger and Jenny Piccolo were DDED.