More complicated weather. In addition to humidity, there would also be temperature, wind speed and direction, atmospheric pressure... and maybe even some conditions that don't exist on earth but could exist on Atys.
Homins would have access to skills or instruments that allow them to measure some of those conditions, and encyclopedia entries that give hints on how to predict changes in the weather. This'd be very usefull as ofcourse the whole world of Atys, materials and plants and creatures and all, reacts to the weather conditions.
More variation in creature behavior. There wouldn't be just herbivores and carnivores fighting eachother. There'd be carnivores that hunt other carnivores, and herbivores that aggressively defend their territory from other herbivores. There'd be carnivorous plants that lure animals into traps, and herbivores that 'hunt' intelligent plants. There'd be animals that live in packs and all fight as one when under attack, animals that live in packs but all flee when under attack, animals that hunt in packs to bring down bigger prey, animals that always live alone, or are even aggresive against other animals of the same species, and many more variations.
More dynamic creature populations as well. Entire species would disappear for the winter as they all hibernate. Other species may spent the winter in the desert but the summer in the forests, migrating back and forth every spring and autumn. Players would have significant influence on animal populations as well, being able to decimate the gingos from a region thus causing an outbreak of yubos there.
Sneaking 2.0! Evading predators will not just be a matter of staying enough meter away from them anymore. Different predators will use different means to detect you, and will require different means to evade. Against predators that use sight, you could hide behind trees. Or stay behind the predator, as long as it's not a species that can see in all directions. Against predators with good hearing you'll have to move quietly. Against a good sense of smell you'll have to stay downwind, or mask your scent somehow. And beware the creatures with more exotic senses like echo-location and heatvision!
I'll come up with more later I'm sure.