Honestly, I blame the voice director for as much of this as the actress, if not moreso. I think the guy had Yuna's VA record her "Okay" line ONCE and then reused it everytime she needed to say that. Horrible.
Spot on. Director's should be given credit/criticism more than they do. Sure, actors/voice actors are the people you see and hear, but every decision is the director's to make. Even the best actors blow many takes, it's the director's job to coax the best performance out of the actor, either by doing it enough times to get a really good take, coaching them, or finding them coaches (nothing to be ashamed of, different rolls take different unique acting research and practice). Part of my job is to do voiceovers or direct voiceovers for my commercials. If I don't like the way someone delivers something, I have them do it over... and over... and over, if necessary. I force them to identify and modify their inflection, stress, and style. At the end of the day, it's my responsibility, and I take the criticism/credit for the overall quality of the piece.
Yuna's voice is a great example. For all we know, the voice actress could be very talented. She certainly has no problems with her voice, itself. But she was given the impression, for whatever reason, to voice the character in an incredibly stilted manner. Also, you can tell that sometimes she just wasn't able to get into the roll, and it sounds forced. I think the localization has something to do with it too (also the director's responsibility). For the most part, Tidus's script wasn't bad, but i think they struggled with Yuna's. It's largely cultural: Yuna's personality is that of a stereotypical demure Japanese lady, for which there is no real western counterpart. The truth is, the localizers should have probably taken a bit more liberty in slightly re-defining Yuna's personality for western audiences. I believe that localizers have a duty to try to make the audiences feel as close to how the original language's audiences did. To a Japanese audience, Yuna's character is recognizable, and probably fairly comfortable. But to western audiences, it becomes awkward and calls more attention to itself than it probably should. Then put it in English, and it becomes even more bizarre. It's then the localizers' job to ratchet down the personality aspects that would make western audiences respond differently to the character than Japanese audiences. Again... director's responsibility. By the time it got to the voice actress, there wasn't a whole lot she could do to smooth out the character. And probably the director was still not thinking so much about western audience's different response to the character, and was pushing the voice actress to be as demure as possible. That was their decision, and I believe it was a bad one. Of course, localization teams have to answer to home base, and it may have been that the Japanese office is to blame for forcing these awkward cultural aspects on the localization project.
That said, I hear that the English voice acting is miles ahead of the Japanese, in this case. I gather they used the motion capture actors to do the voice acting, with very poor results. Many people claim that it's one of the worst Japanese voice acting jobs in video games.