For me, what makes the PT fall apart visually is the lack of grit. Nothing seems real because most of it isn't. It's all very clean, fake looking, sterile and cartoony.

The best example I can come up with for this is the theatre sequence in RotS. Palapatine and Anakin are there discussing the sith and the war, but while I'm watching that scene I can't get the memory out of my head from the making of footage of them sitting in a blue screen box. Virtually nothing in that whole sequence is real, aside from the actors and the seats they're sitting in. Every single thing around them is digitally generated. Why? Because George didn't want to have to build a real set to work on (or, God forbid, go on a real location and shoot there). That's also part of the reason I think that the actors seem bored throughout most of the films. They had literally nothing around them to react and interact with.

Another aspect is that to me the visual designs of the PT look like cheap imitations of Ralph McQuarries' work from the OT. Like somebody described the OT's production design to someone down the phone and said, yeah do that. The designs have hints of McQuarrie's brilliance in them, but only in so much as they are derivative works.