I've been thinking long and hard about this. I haven't made my mind up yet, but I'm going to note down several potential points for now and some I picked up from reading this thread and put together in this post.
NOTE: I haven't got time right now to write in cons for all the others yet. But anybody is feel to add to the list.
I see we have a growing the beard moment thread somewhere. But I can't find it. I saw it a week ago. Nice to pin-point that in comparison with the jumping the shark moment. You'll get to figure out what IS CHARMED for you. With your ideal season being between the growing the beard moment and jump the shark moment.
1) Leo is healed and brought back by Piper and sticks around going into S2. - Love Hurts
CONS: Moving Forward
*More whitelighter mythology which leads to Elders which leads to Up There.
*Damaging Piper's character in the long run.
*The creation of half-white lighters.
2) Andy's death - Deja Vu All Over AgainCONS: Moving forward.
*The show feels less grounded with a mortal/non-magical voice.
*Darryl's character has nobody to bounce off. And has no sense of purpose other than being a plot device for crime-related scenes.
*The sisters lose their grounded connection to someone from their past they all grew up with and new pre-becoming witches.
*The show becomes 70% magical (including Leo's character) and 30% grounded (Darryl, Dan S2, and then 10% grounded in S3 when Cole's introduced), where it was 50%/50% split with Andy in S1.
3) When Prue gets a new power and Astral Projects - Ms. Hellfire
4) When Leo proposed to Piper in the bathroom or When Cole makes the scene of the crime - The Honeymoon's Over
5) Piper's new power when she blew up the watermelon - Exit Strategy.
CONS: Moving forward.
*Less use of freezing power.
*Continuation of a second power for the sisters.
*Piper acting as a Power of One to blow up demons left and right.
*Less reliance on PO3 spells and being creative using their combined powers.
6) - When Phoebe lied to her sisters about Cole - Sleuthing With the Enemy
7) Prue's death - All Hell Breaks Loose
8) When Paige orbs - Charmed Again
9) When Coles eyes turn black - Charmed and Dangerous
10) Wyatt's birth - The Day the Magic Died
11) The Introduction of Magic School - The Legend of Sleepy Halliwell
12) The Introduction of Billie - Still Charmed and Kicking