Here's how I definie 'nostalgia', as per my experience with incredibly ardent SW fans:
1) Complaining about the dialogue in the Prequels while simultaneously ignoring the well-documented fact that the dialogue in the OT is just as bad (if you need proof of that, go watch 'Empire of Dreams', and count the number of times that the 3 main leads comment about the difficulty they had in delivering their lines and taking them seriously. Even lines from the OT that worked - like Vader telling Luke that he's really his father - aren't truly the height of sophistication from a writing standpoint, as evidenced by the fact that even said line has ended up becoming the subject of parody and distortion.
2) Complaining about hokey sequences in the PT while ignoring equally hokey sequences in the OT. I'm a SW fan, but if you were going to ask me what the hokiest thing about the OT was, I'd tell you that it's Threepio constantly getting blown up/knocked apart and having to be repaired. Yes, it works, but it's extremely hokey. Most of the situations that Jar Jar finds himself in in TPM are just as hokey, yet nobody seems to complain about the hokiness of Threepio getting knocked about and falling apart.
3) Complaining about 'flat, one-dimensional characters' in the PT (the Neimoidians, some of the Jedi Council members, etc.) while ignoring the fact that, outside of the main characters, almost everyone else in the OT is equally 'flat and one-dimensional', particularly the Empire's officers. Even Tarkin is a very one-dimensional character at his core.
It should be noted that none of these things detract from my enjoyment of either the PT or OT; they're just simply part of what SW is, yet nostalgia has dulled people's acceptance of that to the point where they pick out flaws from the PT without considering and recognizing that the same flaws are part of the 'OT' as well.
I'm gonna go point by point, I guess.
1. Dialogue is often reflexive of the characters and the type of story. In the case of the OT, they were using variations of mythological archetypes. That's why they were simple. Things like the production design as well as what was said aby the characters about the universe made the universe that SW is in seem much bigger. The characters also worked. They were not deep characters, but they were tangible, relateable, and, most of all, believable. As simple as they came across, it also felt like there was more beneath the surface, and to wach them even in non-speaking moments is to see them thinking, to anticipate how the would react. You know that Han wanted to object to Obi Wan as being crazy long before he actually said it verbally (and, even then, it was only after Chewie voiced his dissatifaction first) This nuance is often lost in the new films, as everything is telegraphed and delivered in a flat cardboard way. We are told that characters in the prequel films are firends, but in the OT, we feel that they are.
I disagree with you. I know from personal experience interacting with other SW fans that, regardless of overall perception of/feeling about the PT as a whole, one of the seminal moments from the trilogy that is often focused on as a positive is the relationship between Qui-Gon Jinn and Ob-Wan, and the way that Qui-Gon's death serves as a catalyst for the way that Obi-Wan behaves, and the actions that he takes. If the PT characters were as flat and one-dimensional as you're claiming, this wouldn't be the case.
Also, I'm going to reiterate what I said to RoJoHen about character archetypes. The same basic mythological archetypes and motifs are present in both trilogies, and many of them are actually carried over between the two trilogies. For example, both Leia and Padme fit the 'damsel' archetype, although neither character is the prototypical manifestation of said archetype, since, as Carrie Fisher points out in 'Empire of Dreams', they do not have the reactions that are typical of said archetype.
Regarding the 'dissolution of tension', I'd argue that this too is a 'flaw' that could be applied to the OT as equally as the PT, even if I don't personally see where the person doing that review is coming from with his perspective.
2. C-3P0 was effeminate comic relief, and, for all his antics, none of the things that happened to him in the OT seemed to fly in the face of phyisics. He was only taken apart n Empire, and for good reason. In the original, we have Jar Jar in cartoon style running to avoid blue spheres by contorting his body like the cartoon he is. We have C-3p0 falling and hitting a flying platform, grabbed by a claw and thrown about as if he is made out of nothing. My least favorite sequence in the PT is the R2 units repairing the queens ship. i know that these movies don't follow science at all, particularly with regards to space, but at least the original trilogy treated flying in space with the same kind of respect as if tehy were flying in the air or anywhere, so, as a result, the R2 units always fit into a slot on the ships so they wouldn't fly away from momentum when the ship went forward, but this sequence, the ship is moving, the droids are rolling around on top and working. That was too much too swallow. For more on what I mean, see here, the dissolution of tension.
I want to specifically address the complaint(s) about Jar Jar, but before I do, I must tell you that I find myself confused by the notion that there is anything wrong with the sequence where the R2 units fix the Queen's ship. In all of my interacting with other SW fans - some of them ardent - I've never heard anyone bring up that sequence as one of the failures of the PT.
Now, about Jar Jar's antics and how they correlate with/compare to the actions of 3P0 and the way the two characters are utilized as 'comic relief', I would argue that the way both characters are used reflects the old 'slapstick' style of comedy. In particular, the sequence(s) involving Jar Jar and the boomas (blue spheres) are meant to evoke the comedy routines of Buster Keaton (particularly the sequence where the 'boomas' chase Jar Jar down the hill, which is an almost exact replication of a bit that Buster Keaton used to do involving barrels).
3. This was covered already. In any film, characters other than the main ones could be considreed flat, unless they are written exceedingly well for what they are, and if the character himself or herself has presence. Tarkin had presence to spare, and through him, we got into what must be the typical mindset of all the Moffs. He inbued this simple role with a lot of presence and we really felt that the Empire was larger than we could ever see on screen. Some of the other minor characters in the OT had presence too, but this is lacking in the new trilogy for background characters. I mean, one bounty hunter hires another to do his job, and he gives her a tube of centipedes, and all she does with it is walk four steps and put that tube into a droid. I ask, why couldn't he do that?
As a writer, I can personally attest that sometimes background characters have no true purpose in the story other than propelling said story forward. Zam Wessel is a perfect example of this. As for why Lucas didn't have Jango do the job himself and omit the character of Zam entirely, we could apply a variation of the same question to the OT with regards to the bounty hunters other than Boba Fett, who aren't even used at all in Empire, or any of the myriad array of Imperial officers and lackeys that we're introduced to. The way that Lucas chose to tell his story and the number of background characters he chose to introduce simply for the sake of introducing them in some cases, and/or for the sake of only using them to propel the story forward, is and should by no means be considered a 'flaw'.
BTW, I was not necessarily stating that the fact that the background characters in the OT or PT are flat and one-dimensional IS infact a flaw; I was merely pointing out that it is generally considered to be such, at least in the case of the PT, but without considering that the very same 'flaw' is also present, should one choose to look for it, in the OT.