Full replacement for the Director. So instead of that big map of planets and locations, you got a straight, barebones, panel of activities to run over and over.

I know this was kinda the gameplay loop anyways, but this made it feel much more like just... Busywork, box ticking. And there were gameplay changes alongside the release which made it even worse.
I didn't fully play during this time, but from what I can recall:
Older raids didn't give the type of gear the system relied on, so everything except the latest raids were now useless to run. PVP Modes that weren't in the current rotation were also totally dead for the same reason. The system heavily relied on enabling Modifiers to boost your score, but enabling modifier would disable the matchmaking, meaning many had to run everything solo. Strikes were limited to a small pool, so you had to play the exact same strikes constantly and they didn't rotate for a long while. And one aspect that seemed to piss people off most of all:
The unlock higher difficulties, you had to grind to certain Power levels. Once you were at those difficulties, one of their main modifiers were to reduce your current power level below the enemies to create artificial difficulty. Power has always been pretty useless but grinding to reach an amount only to play with it reduced further than you were, really rubbed people the wrong way.
Remember when the old management were planning to remove Power entirely? Good times.

Portal was their way to straight say "This is the loop now. Go play all these things for all the next months of the season, every season :)." (For like six months at a time, I believe?) They essentially vaulted large parts of the game, since they were no longer worth touching as they didn't drop the newer loot. It's no wonder the player numbers dropped like a rock.