azira says:
I think defences vs saves boils down to "players like rolling dice". It's the same outcome, but if you save, you roll the dice (as opposed to the other guy), and you feel like you're in control of the outcome (not the other guy).
I understand this perspective, and I think I remember reading it as the reason they gave for why it was changed. However, I think that was just a dismissive answer that they knew people would accept instead of the actual reason. Let me explain.
For starters, NAD are a better mechanic design-wise. NAD work exactly like AC. "I want to hit that zombie with a firebolt. I got a 14 vs Dex." DM says "The zombie moves too slowly to avoid the mote of fire that you flung at his chest. Roll damage." You see how that conversation is the same as attacking vs AC? It's a mechanic that people know and can figure out intuitively. That's good design.
The conversation is different for saves. "I want to hit that zombie with a firebolt." DM says, "What's your DC, and what's the save?" "Player says, "Um, it says Dex save, but I don't know my DC. Don't you tell us what a DC is when we roll something? Why am I picking the DC? I mean, if I'm setting the DC then it's 99." DM then has to explain why DCs are different in this one instance compared to how they are used everywhere else. DM also has to explain how to calculate the character's DC when it casts a spell at something." It's not intuitive, and anyone that has played 5E has had that conversation before - with people completely new or people that have played previous editions.
Another benefit of NAD is that they make more use of ability scores than saves. Saves in 5E go directly to ability score modifiers themselves, but the same powerful abilities are prevalent as in other editions - Dex, Wis, Con. The outcome is a heavier value on those abilities when making any character. In 4E (which used NAD), people could select ability scores that meshed well with their character concept and not sacrifice survivability.
Specific to the rolling rationale, AC would not be in the game if they reached the conclusion that saves are better because people like to roll. Armor would, instead, provide bonuses to Physical Saves or STR ability checks. As explained above, the better design is the one that is intuitive. In that case, NAD don't just go away. So does AC. I think it's obvious why they didn't change that - people would revolt. AC is an iconic part of the system.
And that's where I think we touch on the real reason they removed NAD. They wanted to create a new system that is closer to the iconic system that people remember from the past. They were pandering to an audience. Or as Naatkinson mentioned earlier in this thread, they were cashing in on nostalgia.
I know I'm in the minority, but I think the 4E hate was misguided and if people are being fair, then they would see so many pieces of 4E that were great design choices. I listed the ones I liked best in my previous post. Still, 4E did have some aspects that were good choices but were poorly executed. Class powers - great idea, but the powers themselves were often easily broken or exploitable. Plus, the language was not clear enough on most, if not all, of them. Twin Strike in PHB is a good example. It was also too feat heavy, and some feats were just ridiculous. I'm looking at you, White Lotus feats.
I believe the real reason that they dropped NAD from the system was because they wanted to more closely mimic the older iconic system and also because people unfairly hated on 4E even though the 4E materials sold like hotcakes (who doesn't love hotcakes?). Of course, they can't openly say they are cashing in on nostalgia because that is like saying "We're repackaging something you already own in hopes that you buy it again." So they sell it on the "fun factor' and people don't often give it much thought after that.