The DE of TMP was just updating the effects, in fact I would say that was secondary, mostly fixes and some enhancements.
But it also fixed some of the pacing issues of TMP. So some scenes were removed and edited for pacing purposes. Sometimes directors cuts are not necessarily the director's preferred version, sometimes the director intended on the theatrical version to be their definitive version (i.e. Alien by Ridley Scott). But in TMP's case, Robert Wise never seemed entirely satisfied with the theatrical cut, or even the special longer version released on VHS. I think that's why it's called a Directors Edition instead of cut because this was a version Wise worked on to fix what he saw were some flaws (though not necessarily a final edit that he would have done in 1979).
Pauln6 probably has the best place to find the differences though.