I've been thinking a lot about this all afternoon/evening (being in Australia, the news hit right at lunchtime at work for me).
On the positive side: I think it's a net positive for the game to reduce some of the odd interactions between legendaries and Pacifism-style effects. The fact that Pacifism effects double as Meddling Mage effects against legendaries isn't something I believe was intended. And it's certainly a corner case, but preventing feel-bad blowouts with blinking Journey to Nowhere effects is a positive as well, in my opinion.
I also like that this means that drawing multiple legendary creatures isn't as bad of a thing. You still don't get to have two of them, but if something happens to the one on the battlefield (say it gets a bunch of -1/-1 counters on it), your additional draw can act as a re-buy. I feel like it makes flavor sense, and prevents a lot of feel-bad moments. Which I think is overall good for the game.
What I don't like is the interaction with your opponent's board. I don't like that multiple of these creatures can exist on the battlefield at any given time. Taking that drawback away from Legendary creatures isn't
necessarily a bad thing in a vacuum, but so many overpowered creatures have been printed with the justification being that "they're legendary" that I would be very afraid of doing this.
Regarding Planeswalkers: I really, really like the effect that this rule has on my interactions with my opponent. I'm no longer punished for choosing to play the more expensive version of a planeswalker. I'm no longer punished for losing the die roll. Under the old rules, if my opponent got the walker off first, even if I removed it with mine, he still got an activation out of it. Planeswalkers don't have summoning sickness; games that involve a race to get out the first walker are
not fun.
I also think (and I mentioned this earlier) that this opens up design space for 2CMC planeswalkers. Imagine if they printed a 2CMC Jace right now. He could have no abilities on him, and he would still be a "2CMC Sorcery - Destroy target Jace" and "2CMC Enchantment - Your opponents can't cast Jace." He's already playable
without any abilities. This means that you can't make his abilities very good. But by changing this rule, you're free to make 2CMC Jace a good card without worrying about the other stuff that comes with him.
But what I hate about the new rules is getting to buy extra planeswalker activations by spending a card. I can just imagine a Modern Jund deck playing a T2 'Goyf, T3 Liliana (edict), T4 +1 Lili, cast another Lili, +1 again. Or a Modern Tron deck freely exiling your best thing before resetting Karn and taking something else. Maybe that won't prove to be too overpowered, but I feel like it's going to create some incredible blowout states.
TLR; I think the rule improves creature interaction on your side, but makes the interaction between you and your opponents worse with regards to creatures. And I have basically the opposite opinion with regards to planeswalkers: I think it makes the inter-player interactions better, but buying extra actions on a turn seems dangerous.