Lucas was so caught up in his own universe and ideas that he forgot to focus on making a good film. The original Star Wars borrows a lot from older movies, particularly the influence of Kurosawa. The plot of ANH is almost flawless in structure for an adventure film.
No good characters, no humour or fun in the adventure. No one we identify with or want to be like (solo!)
Jedi's seem really boring. Not like Samurai at all. They are a bunch of fools who don't see the obvious. And they all twerl their light sabers too much.
Flat performances from almost everyone.
The decision to start the story on such a young Anakin.
Obviously the over reliance on green screen and cgi, making most frames a cluttered mess. No such thing a economy of statement in these films.
Linking too many things together - Vader building C3PO, Chewbacca being Yoda's friend. This all makes the universe seem smaller.
Ruining the Force.
Just really poor handling of almost any scene. Anakina and Padme's romance - dreadful, Anakin's final motivation for turning to the dark side and killing other jedi's.... So he could bring back Padme when she dies during child birth - how about just telling a doctor instead - maybe in secret! He's an idiot.
The fact that nothing looks or feels like Star Wars. I'm hoping that with the new trilogy they just loot all the designs from the OT. I want to see AT-AT's and Tie Fighters dammit!
The fact that the films were made at all. Prequels are never a good idea, as if a film is successful enough to warrant a prequel it obviously means it doesn't need one. They weakened many things from the OT, particulary Vader that you just have to forget about them. To me, Vader didn't built C3PO - it's stupid.
There are loads of other things, but it's depressing typing this, so I'm going to stop.