water physics including flow and the water pushes your character with the current.
land deformation and destructable terrain (if youve encounted any creepers you know what im talking about)
advance snow physics. not only can you collect it but you can also craft snowballs with it.
one of the most complicated and versatile crafting systems ive seen ever including in MMOs.
for you fellow engineering students out there they have red dust which is used for creating on/off switch systems for doors and various contraptions
intelligent AI. if a creeper cant get into your house and you get to close to him to swing your sword he will simply blow up and kill you or at least take out your door and windows. i learned the hard way that you cant have to empty side by side space open no matter how high you are.......those spiders will get in. eeks
fire actually burns the environment and even spreads further out.
gravel is affected by gravity (i wonder why they dont do the same thing with dirt)
tools and weapons degrade with use
an exciting plotline similar to castaway with tom hanks
probably lots more things i can think of.