The prequels had a stellar cast. Really. It's hard to think of a franchise with more gifted actors who were famous before the trilogy was made. In fact, I don't think there is such a series. Maybe the Chris Nolan Batman films but that's about it.
-Ewan McGregor
-Liam Neeson
-Hayden Christensen (was known as "the next big thing" around 2000 when cast)
-Natalie Portman
-Christopher Lee
-Jimmy Smits
-Terrence Stamp
-Samuel L Jackson
Plus you had the fantastic but unknown to Americans Pernilla August, Ian McDiarmid who people didn't realize was as good an actor as he was, and also that girl from Whale Rider is in there. You even have Sofia Coppola and Kiera Knightly in the mix! And Temurra Morrison, who was a big deal in the indie scene.
And wouldn't you know--most of them stink. The casting director--Robin Gurland I think--did an amazing job. Directors would kill to have a cast like that at their command. You could have made the best-acted fantasy film ever made with a cast like that. Instead, almost of all them go down the toilet.
When I was in film school, we had to take acting lessons as part of our directing exercises. And one of the things we learned about was what is known as "self-directing." This can be a problem for some directors, which is why we were learning about it, because sometimes a cast member will just ignore your direction and make their own choices. Well, this is also a skill that actors can use to get themselves out of trouble when your director doesn't know what he or she is doing--and, like real directing, it's an acquired skill that takes years of experience to do well.
No surprise that all the older actors faired pretty well. Christopher Lee and Ian McDiarmid are old enough to direct themselves when George Lucas just stands there and says "read these crappy lines faster and more intense." But the younger cast--they have no clue. They just do the work and listen to what George says, because they trust him and don't know how to direct themselves. So, if you look at the prequels, it's mainly the over-40-years-old actors that managed to turn in something not too embarassing.