One of the Yoda's quotes which sticks in my mind most from the original series is
A Jedi uses the Force for knowledge and defense, NEVER for attack!
To back this up we see Vader and Sidious using the Force against Luke, Vader throws things against him to wear him down and toy with him, throw in some Force chokes and he's definitely not following Yoda's teachings. Sidious uses the Force Lightning.
I've always assumed that using the Force in an offensive manner was banned for Jedi because it was a path to the dark side, to use the Force to attack rather than defense was to try and control it - very Sith-like.
However when we move onto the prequel trilogy we see the Force being used offensively very frequently (admittedly mostly against droids who may not be considered "alive"). The example which springs to mind is where we see Yoda throw the two red guards against the wall when he confronts Sidious (not to mention some rather offensive Force moves in the Yoda/Sidious and Obi Wan/Vader duels).
Why does Yoda forbid Luke to attack with the Force when it seems such common practice thirty years before?