As a long time Star Wars fan, I've had the same reaction as many others: I still love the originals and don't much care for the Prequels. The problem I see, however, is the argument that "the Prequels were only bad because they were over-hyped." While it is true that they could never live up to the expectations of the Originals, hype isn't the only factor that comes into play when analyzing these films. As a writer and quasi film-junkie, I've spent the last 15 or so years trying to figure out what makes a story a successful story. Disregarding financial success (in which case, all of the six Star Wars films would be considered successful), the biggest differences between the Prequels and the Originals are the realistic motivations of the characters.
Let's take Han and Leia vs. Anakin and Padme for an example. Han is a charming, confident, roguish scoundrel and Leia is a strong-willed, take-charge, no-nonsense leader. Despite the initial friction and differences between the two, they end up together. Their romance feels natural and realistic because separately, they have individual character motivations that we can relate to and admire. We root for them to get together, and we root for them as heroes, because they're written realistically enough as to remind us of people we would want to know and befriend in real life.
In contrast, let's discuss Anakin and Padme. Anakin is a whiny, creepy, rebellious, rage-and-hate-filled teenager. Padme is a pushover. She's also dry, doll-like, bland, boring, and... (I can't think of other synonyms for "bland"). She's a damsel in distress and a liability. While the argument could be made that this makes them realistic, the problem is that it doesn't make them realistic heroes. If there was an actual "War of the Stars," I wouldn't want these two leading me into battle. I would want Han and Leia, because of their strong leadership qualities.
Aside from the hero aspect, there's the romance aspect. Though it could plausibly be said that these two characters might end up together; on screen it feels awkward, forced, and uncomfortable. To me, it's the equivalent of the two weird kids in my junior high class hooking up. More power to them, but it's not a story I particularly want to hear about, much less take two-to-six hours watching.
That's the main difference I see between the new films versus the old: the characters in the new ones make decisions based on weak or confusing motivations. They just don't feel like characters that I would want to know or care about, and therefore I've never been able to become emotionally invested in the Prequels.
One final thought: the characterization problems I've discussed above are aspects of writing. In my experience, good writing will always trump visuals as far as a good story is concerned. Impressive visuals only enhance a movie if it has a captivating and engaging story as a strong foundation for those visuals. To quote George Lucas himself: "Special effects are just a tool, a means of telling a story. People have a tendency to confuse them as an end to themselves. A Special effect without a story is a pretty boring thing." I wish George had followed his own advice when making the Prequels. I really do. Unfortunately, he didn't and we have the Prequels as what they are - flashy special effects without a well-written story to give them substance.
Here's hoping that The Force Awakens does a better job. My prediction: it'll be better than the Prequels, but still won't fully capture the magic of the originals (a Herculean task). We'll see. :)
P.S. If you want a short synopsis of why the prequels had problems, I would suggest the following two videos (For Episode II, at least).
How It Should Have Ended: http://www.howitshouldhaveended.com/.../how-attack-of.../
Honest Trailers (WARNING: Language): https://www.youtube.com/watch?v=4nJS-LPcFfw