As anyone who knows me can tell you, once I start down the "gushing" path, forever will it dominate my destiny.
Anyway to get things started, in a nutshell, the reason I think the full saga is better than either of the trilogies on their own is because of the way they mirror each other. Some people will say Lucas is just bereft of ideas, which is why we got a 2nd Death Star in ROTJ and a similar "blow up the space station" sequence in TPM. While all three of these "blow up the station to save the day" scenarios are indeed similar, closer inspection reveals the differences between them that elevate each of them from just being a carbon copy element.
First of all it's widely known that Lucas was originally going to just have one Death Star at the end of the classic trilogy story, but moved that scenario to ANH to give that film a huge dramatic punch at the end. When it did well and he knew he could finish the trilogy, he came up with the 2nd death star idea. The differences between the 2 Death Star scenarios are that first of all, it's the ancillary characters who are "saving the day" in ROTJ while Luke Han and Leia all have their own separate roles to play elsewhere. So it's sort of like the new generation of rebels, inspired by the heroism of Luke Han and Leia carrying on the torch of fighting freedom.
Another difference is that while the first Death Star attack was pretty simple and straight forward (the plan was always to hit the exhaust port while the imperials defended), the 2nd Death Star attack plays against the expectations of the first Death Star attack by making the whole thing an elaborately planned trap by the Emperor. Truly showing the scope of his power to influence everyone around him, he controlled the rebels right into his trap with misinformation. So the rebels go in with a similarly simple plan (albeit now they have to fly inside the Death Star to do it) but then the tables are turned when they realize the shields are up and the Imperial fleet was just waiting to corner them in. To me that made some serious drama on it's own when I first saw ROTJ. When Admiral Ackbar said "ITS A TRAP" I was going nuts on the edge of my seat.
Okay so that's what’s different about the first 2 "blow up the space station" bits. Now with TPM, we have another situation where the Skywalker is the only one who was able to hit the reactor to blow up the ship and save the day. It certainly mirrors Luke's run on the first Death Star more than it does Lando's victory in ROTJ. The big difference between them though is that Luke was consciously trying to hit a target, while Ankain seemed to "accidentally" hit the buttons at just the right time. Just before Luke fires his shot, he is urged to "use the Force" to make it in and Luke does that by consciously tapping in as Ben had taught him.
In Anakin's case, he's not trying to use the Force, but at the same time, it's pretty miraculous that he would just happen to land facing that particular direction, and he would happen to try the torpedo buttons at just the right moment.
To me this is illustrating the 2 dynamics of how the Force works.
Luke: You mean it controls your actions?
Obi-Wan: Partially, but it also obeys your commands.
I think Luke used the Force, while the Force used Anakin. So in that way I don't see this as a re-hash, as much as showing the other side to the same coin. In that way each scene augments the meaning of the other by offering contrast to these ideas.
Midichlorians
Lucas talks about how he always intended there to be this more scientific side to the Force, but that he just didn't get into that much in the classic trilogy (I think I am getting this from the commentary on the TPM DVD). If you read the ANH novel (even before the SE's came around) there is a part where Obi-Wan talks to Luke about how the Old Republic tried to define the Force with science, but that they never quite could. That perhaps the Force is just as much magic as it is science. Not more magic than science but just as much. Add to that the way Luke was strong in the Force because he was the biological son of a powerful Jedi and it's easy to see what Lucas is talking about with regards to the "science" of the Force even back then. So all this stuff about how Lucas took away the mysticism and replaced it with science isn't entirely accurate.
Now the concept of Midichlorians themselves is fascinating to me. Lucas loosely based them on our real world equivalent: Mitochondria. Just like Midichlorians, Mitochondria are a microscopic life form that lives in every living cell here on Earth. Scientists theorize that they are the reason life exists in the first place and that without them, we would have no knowledge of the Force (well okay maybe not that last part but these two concepts really are that close).
The really fascinating thing about them is that when scientists started studying Mitochondria, a lot of religious people were saying we shouldn't be studying them. That finding out the source of life could stand to disprove God himself. Just as many Star Wars fans were insisting that the introduction of Midichlorians could stand to de-mystify the Force concept.
In reality neither do any such thing. Neither the study of Mitochondria, or the introduction of the Midichlorian concept "explain" God or the "Force" respectively. They both merely add a whole new layer of questions on top of the ones we already had.
Some fans act like the Midichlorians fly in the face of everything we know about the Force in the classic trilogy.
For example, some people say if Midichlorians are only in living things, then how could the Force be in the rock as Yoda says in Empire? The answer is that Midichlorians aren't the Force itself; they are merely antennae which allow 2 way communication between living things and the Force.
Other people say that Midichlorians suddenly make the Force biological, but as I explained earlier, it was always something passed down from one generation to the next as exampled by Luke being strong in the Force because he's related to Anakin. Also the very concept that they would be able to make the Jedi "all but extinct" shows that Force sensitivity would be something that is genetic rather than purely spontaneous.
I could go on and on like this aimlessly, but I want to hear your questions if you have any to point me in a more meaningful direction for you.
For those of you who really didn't like the Midichlorians, please bring up concerns I have yet to address.
(edit I spellchecked)