TPM is interesting to me as a reflection of who the filmmaker was when he made it - GL seemed much more interested in the sociology of the SW universe in TPM than he did in the OT. I find a strong undercurrent of galactic caste, as someone in an earlier post pointed out. There are simple statements that speak about the bigger problems of the galaxy and it's society - Padme's ignorance of slavery, Padme and Nass's mutual dislike/distrust/ignorance of each other's species, the infighting and corruption of the senate, the beauracracy of the corporations and their influence on the machinations of politics. Although it's never played out in the PT films, one can imagine Palpatine's frustration with the inneffectual Nemoidians (and subsequently the Geonosians and the leaders of the Seperatist council (all aliends by Ep 3)) leading to his overall dislike of non-humanoid beings.
There is also a strong theme of duality in the film, which becomes a major motif in the saga; for example - Padme has a duality in the persona of Queen Amidala, Palpatine has a duality in the persona of Darth Sidious (although the story doesn't overtly reveal this); there is a duality (symbiosis) that must be struck between the Naboo and Gungans if they are to survive; there is a duality (symbiosis) between the midichlorians and all living things (one thing that I find fans tend to forget when they criticize the midichlorian issue is that Qui-Gon states the EVERYONE has midichlorians in their blood stream, but some have more than others - it doesn't follow that midichlorians ARE the Force or that it contradicts Ben's description of the Force in Ep 4 - it's just a scientific explanation rather than a spiritual one. Does anyone think Han Solo was particularly strong in the Force, or could become so with or without midichlorians?). Anyway, this theme of duality reaches it's ultimate peak and simplicity in the balance of Good and Evil in Anakin - once in ROTS when he decides to kill Mace and join Palpatine and learn the ways of the Sith and again in JEDI when he decides to betray Palpatine and throw his down the shaft. There is duality in the training of a Jedi, and it turns out, a Sith - master and apprentice - always 2 there are. There is even a duality to the Force itself in Ep 1 - the living Force versus the other Force (I forget what its called at the moment, sorry). Throughout the saga, this duality continues on in Princess Leia (Imperial Senator/Rebel Leader), Han Solo (scoundrel/hero), Ben Kenobi (crazy hermit/Wise Jedi), and Darth Vader (machine/man).
Watching all 6 films in numerical order, TPM is an unusual entry - it's separated from the others by such a long time (10 years), and the characters who become central to the story play fairly supportive roles (Obi-Wan and Anakin), leaving center stage to Qui-Gon and the Queen, one of whom for all intents and purposes exits the saga by the end of the film, while the other becomes less and less important until her demise. There are many moments of symmetry between TPM and ROTJ, such as the funeral pyre for Qui-Gon (echoed in Vader's pyre in RTOJ), the Pod Race (echoed in the speeder bike chase), the short of Obi-Wan radling a dying Qui-Gon next to the bottomless pit (Luke cradling a dying Vader next to a bottomless pit), the helicopter shot that introduces the Jedi Temple (the helicopter shot the introduces Emperor's throne room on the Death Star 2). I felt Lucas succesfully bookended the Saga with many visual elements. Certainly the Gungans and the Ewoks mirror each other both in their conception and in their unpopularity with fans.
Sadly, Luke and Leia become smaller characters in the PT when it's viewed in the context of the 6 films, since their overall dramatic purpose is to undo the damage Anakin caused; Anakin becomes the overarching focus of the six films (despite his supporting role in Ep 1.) One interesting re-emergence is the feeling at the end of Empire that Luke really could turn to the dark side in Ep 6 (if you pretend not to know what's going to happen in JEDI) - having watched his father fall from good, it becomes that much more believable that the son would fall as well - although you see that without a "dying wife" (a potent carrot on a stick) to use against him, Palpatine is not successful in corrupting Luke as he did Anakin - he must instead resort to threatening speeches and clumsy attacks against his friends.
Also, the dramatic structure of TPM mirrors the structure of ANH - in TPM Qui-Gon and Obi-Wan lead us to Padme who lead us to Tatooine and Anakin, which gets us to Coruscant which ends up sending us back to Naboo to destroy the Trade Federation blcokade/droid army. In ANH, the robots lead us to Luke who leads us to Ben who leads us to Han who gets us to Alderaan (almost) which helps us rescue the Princess which gets us to Yavin and gets us to destroy the Death Star. (EP 2 also mirrors Ep 5 - Anakin and Padme go off together in fall in love (a la Han and Leia) while Obi-Wan searches out a mystery on a strange hidden planet (Luke goes to the mysterious Dagobah) then the 3 are reunited in a climatic battle (Han, Luke and Leia all end up on Cloud City for the final act). Oh, and the bad guys win in both films too. Ep 3 mirrors Jedi to a degree - the opening act of both films is a rescue mission for an important character (Palpatine/Solo) while the closing act centers on a climactic sword fight between 2 "brothers"/father and son.
People often dismiss the Trade Federation taxation storyline as too small a story for the film, and perhaps they are right in terms of TPM being a stand-alone film. But I don't think TPM is truly intended as a standalone film - it was concieved as the opening chapter in a 6-part saga. I like the fact that the origins of the Galactic Civil War are traced back to this seemingly unrelated, arcane trade dispute. That's a realistic notion when one examines the true history of our society. (e.g. the assasination of Archduke Ferdinand triggering a World War). I think many of the criticisms lobbed at the plot of the film could equally be lobbed at the much love FELLOWSHIP OF THE RING if you examine it as a stand-alone film - the battle of Helm's Deep from TTT is a much more critical and important battle than the one that closes FOTR, but FOTR has to tell the story it has to tell as the opening chapter in a saga - and people seemed to accept that going in. In this regard, I think TPM gets a bum rap.
Hey I know I'm not gonna change anyone's mind if they hate TPM, but I hope people enjoy reading about the good things people see in it.
For what it's worth, for me ROTJ is the weakest of all the SW films - the complete castration of Han Solo's character is a disappointment, the low-rent death of Boba Fett is a major disappointment (especially considering the buildup the character gets in Ep 2), the Ewoks are on par with the Gungans for their dislikability and supposed cutesy-factor, and perhaps most offensive is the utter dismissal of any leadership qualities on the part of Leia - she literally becomes eye candy for the first half of the film and becomes the lame-duck Han Solo's underling for the second. In the "humor" department, I would characterize the torturing of a robot as ridiculous as anything in Ep 1(well, ok, not the ick icky goo or fart jokes) and Palpatine's characterization by McDiarmid, although enourmously enriched by his work in the PT, borders on the mustache-twirling scenery chewing of a bad B-movie (with echoes of a pedophiliac Mr Burns thrown in for good measure). That isn't to say TPM doesn't have it's shortcomings - it certainly does, but somehow I can accept them since TPM is the opening chapter in a six film saga. ROTJ is supposed to be the big finish to the entire story, and we get half a movie devoted to the Muppet show (complete with musical number), and half a movie following teddy bears around a forest while the Empire acts like buffoons and the Emperor says creepy, vaguely sexual things to Luke before dying from a bad fall. And to finish the whole thing up? A big campfire singalong! Yipeee! Wizard! That isn't to say ROTJ doesn't do things right - I think all the interplay between Luke and Vader works very well and was only deepened by the PT, and the special effects in the space battle are perhaps the ultimate achievement in the quickly dying art of non-digital, motion-control optical compositing.
Finally, it's interesting to me to see how closely Lucas kept the PT to the opening prologue of the original novelization to Ep 4, written by Alan Dean Foster in 1976 - check it out next time your at the bookstore.