Omega 
I'm not fond of brushing off people who don't like modern Minecraft as being blinded by nostalgia. I do think there was something special about old Minecraft and there is a reason why a lot of people keep playing on beta version 1.7.3, to the point where this version has an extensive and quite active modding scene now.
But looking back and wondering what it is, I think it's the worldgen and the color palette. I started playing in beta 1.5_01. I was there when beta 1.8 released and I immediately noticed these two changes. The grass got more muted colors, more realistic ones, and the terrain got flatter. All though the terrain generation has changed quite a lot, it still doesn't have that strange magic that beta pre-1.8 had.
That terrain generation, to a lot of people, myself included for a long time, that was Minecraft. If you've never experienced or seen it, it may be hard to understand what I'm talking about but for me, it's almost instinctive. It's a "I know it when I see it" thing.
I'd love to get a mixture of both. Maybe have the overworld of Beta with the new biomes mixed in to all of that and keep the new cave system. I'm kinda convinced that if that part of the game as well as a more vibrant color palette had been kept, you wouldn't see such feelings for that era of Minecraft.
MODPURE
Betacraft
Eter (´= ̫=`) 💙💛
Undead Zeratul
Sparky