hmm.... tbh, i could put almost anything here, especially as someone who makes resource packs, but for my first one, i'm gonna go ahead and say water.
not only does waterlogging work more like how one would expect it to (with the exception of redstone components, which can be partially waterlogged for some reason), but it's also not arbitrarily limited by block type. so long as a block isn't fully solid, it can be waterlogged, and the water in that waterlogged block will flow the way you'd expect it to instead of just staying there awkwardly.
also, there are just more colors of water. sure, water color is biome-dependant on both editions, but java edition uses the same, eye-piercing blue for almost every biome, while bedrock instead has a unique shade for almost every biome. it really makes the water feel more like it belongs in whatever biome it finds itself in, and also it just generally looks nicer and more interesting. i'd probably site old growth taigas as an example here, but somethings tells me most people don't find them as pretty as i do.
and, if we're considering resource pack functionality as a feature, both the color of water and the opacity of water can be changed, per biome, via resource pack.
