The world gen is way better. It's shocking how much better it is, they did a really good job. The caves are more interesting, the biomes blend better, mountains are taller and more impressive. The old gen is still nostalgic, obviously, but I'd much rather play with the new worlds.

There's a bunch of new blocks that are nice, including some redstone stuff. There's some new mobs, and lots of small things I'm forgetting. Honestly, on the whole, it's been a good few updates.

It's not official, but the new horizons mod is insane. Literally hundreds of chunks of render distance, with very low/no performance hit compared to vanilla. It's incredible.