Computers (auto-pilots, LNAV/VNAV) don't take-off.
.....they don't know when to shut an engine down.
.....they don't put in aileron or rudder trim. (at least on a Boeing)
.....they don't know how and when to change altitude for turbulence.
.....they don't know which way to go around a storm.
.....they can't predict traffic conjestion based on radio traffic.
Here's the biggies....
.....they can't land with an engine failure.
.....they can't land unless there's a precision approach available to that runway.
.....they have lower wind tolerances for landing.
.....they can't fly visuals, a la the "Expressway Visual 13" @ KLGA or the "River Visual 19" in KDCA, or even a straight in visual.
.....they can't judge rapidly changing conditions, either mechanical, weather or ATC systems.
.....they can't double check to see that dispatch gave you a suitable alternate and/or enough fuel to get there.
.....they can't double-check to see if the logbook/MEL's are properly signed off.
.....they can't coordinate with emergency medical services.
.....they can't be flexible with ATC to make the system work.
.....he!!, they can't be flexible with anyone to make the US air transportation system work.

Question: If you have VNAV in your plane. How well does it work?