In The Empire Strikes Back we see Yoda as a mysterious and powerful Master. In The Return of the Jedi we see that Yoda could be wrong. And now, in Revenge of the Sith, we see Yoda as a blundering fathead. In the pivotal scene, when a miserable Anakin is looking for advice about his wife, the great Yoda delivers a Buddhist sermon on the virtues of detachment.
I guess this is supposed to tie in with the whole vague business of "balance" (the Jedi clearly didn't have both oars in the water if they'd countentance things like "truths we cling to depend greatly on our point of view"), but I can't be bothered to work out continuity issues for Lucas. He swears there were to be no movie sequels, but one scene leaves an opening for sequels, or for a TV series. . .
Lucas is very good at spectacle, and he delivers. The dialog isn't quite as bad as I'd expected, but the poor dialog and vague acting completely undermine what should have been a climactic scene when Anakin swears himself to the Emperor.
And Lucas tries to lay the foundations for the rest (first) of the series and tie things together, and does a pretty decent job of it. He's never worried too terribly much about continuity before; making sure the plot runs fast has been more important. And so, dark tone or no, it is a pretty fun movie.
But for crying out loud: After nine hundred years, Master grammar Yoda cannot?