Which special effects CAN'T be done?

The way the CGI Morlocks moved in the newer version of The Time Machine is a perfect illustration of the “lack of mass” problem. Things of a certain size and weight have to abide by the laws of gravity. Laws that computer animators are apparently unaware of.

On the other hand, the dinosaurs in Jurassic Park seemed to move in accordance with real-world physics.

I’m not so sure. There’s probably a lot of films that use CGI smoke, water or other fluids that you just don’t notice.