There could be alot of reasons that only the most basic animations and basic physics were used in the showing, at least from a developers standpoint.
Could be the fact the demo Yoshi played with was merely just that a DEMO that probably was hastily put together by the developers in order to make the deadline for the con, so they left out some of the immersion features.
Or perhaps the demo Yoshi played was made before the reworked physics and animations were fully developed.
Or the demo was buggy too buggy with the detailed animations and physics therefore left them out to avoid community backlash or problems during the presentation. (Which is a smart move IMO if that was the case)
Or the demo was not connected to a test server and was fully installed within the computer presented, therefore leaving out all but the most basic functions and UI features.
Or just bear with me now, the new server structure still requires the physics engine to be reworked, and they may be still in the process of doing so. Which I could see happening, even if they have the math part done doing little things like the grass blowing in the wind, slowing down your pace from a full sprint, or getting knocked off your feet can be taxing on the games resources and still requires it to be fine tuned.
There are a plethora of other reasons they may not have included it, but from a general standpoint it's too early to tell what's really gonna be implemented in the game.
