Well speaking for Maokai:
Passive change feels mostly the same, I feel like it's to somewhat try to make up for Maokai's tankiness if he charges in now because of the ult change.
E change is a big neon sign for the utility and vision control in those saplings. Down here in low Silver, the few times I've seen a Maokai have never had saplings in brushes.
R change I think is actually great. Maokai's old ultimate wasn't actually that large in that if you position aggressively and manage to get in the enemy's back line, or if the enemy front line is more mobile that Maokai, the ADCs (the ones I'd want to protect the most) are still exposed. Maokai's not actually doing that much to drag attention to himself, as he lacks the damage of a Juggernaut and he's not slamming cc into the enemy like Nautilus literally every auto attack. Defensively it works wonders, but from a thematic standpoint it's incredibly boring. We have Shen teleporting from across the map, we have Braum standing in the way shielding damage, we have Tahm devouring the ADC and running off with them, so on and so forth there are Tanks with such interesting ways of defending their ADC, and Maokai does it by standing in their general area.
The new ult does two fold: as a slowly progressing line of CC, if you initiate a dive by ulting, running in front of it, and then diving whoever you can, you're still defending your back line as the ultimate creates a virtual wall between you( the enemy team), and your carries, while at the same time the slow travel speed isn't actually that bad when trying to defend your carries and creates a huge zone control tool to defend them with.