While I understand that you want to avoid another heated argument about specific cards, I think it's impossible to discuss criteria for ban worthy cards without using examples.
That being said, I think FOGP brings up a 5th criteria that should be accounted for.
5. A card that can limit or control the design freedom of future cards in a certain archetype due to the potential for generating unforeseen or excessive advantage for such archetypes.
While we never have to worry about seeing FOGP in standard anymore, if that card were to remain in expanded, the design of future Grass Pokemon would either need to be stifled to accomodate for something as powerful as FOGP, or grass decks would just run rampant in expanded simply because they have a significant tool no one else has.
Just imagine if FOGP was still legal with the Venusaur/Shining Genesect deck floating around nowadays. Or before that, Golisopod GX in expanded - open Wimpod in active, Brigette for some more, FOGP into Golisopod GX on bench, attach energy, wimp out of active, switch into Golisopod GX, and deal 120 damage on your first turn (going second) of the game. Lurantis GX/Venusaur starts to look like a super viable deck if FOGP was still available.
Between banning all of the good Grass Pokemon and never designing anymore good Grass Pokemon, or just simply banning FOGP, simply banning FOGP was clearly the better decision. They simply cut it off at the root of the problem so no one ever has to worry about Grass Pokemon as a whole (get it, Grass Pokemon? Roots? You can punch me if you want).
The wording of my point can definitely be improved, but I think the general idea can be understood. FOGP made every Grass Evolution Pokemon better than it is simply by having that card available to them.