We have come along way when it comes to computers and graphics, but there are some stuff that havent been done properly (or at all) I will list some
1. Running water
Imagine liquid, Imagine a world of blood, water, oil, lava, mud, slime...you get it...is it really that hard?
2. Clipping problem
Its not a big deal, but seeing how a hand go right tru the neck when the character scratching his back makes it looks alittle "low budget" dont you think?
3. Fire
same as with liquid, have been done 100th of time but the only fires that caught me was RTCW's flame thrower and Postal 2 can with fuel.
......... come up with your own! Maybe santa will bring em to you