Let me start out by saying that Jedi isn't horrible, and if placed next to the prequels, it begins to look like Lawrence of Arabia. But it does have a number of flaws which were, had we but known, omens of problems which would burgeon to gargantuan proportions in the next films. Before starting in on my criticism of the film, however, I'll point out a few things about the movie which are fine.
So those are my thoughts on the first part of Return Of The Jedi; more later on the rest of the film.