I don't think it's mediocre, it's just plain bad. The lengthy, unskippable cutscenes completely ruin the flow of the game. The sphere grid is unnecessarily complicated. The characters are annoying, particularly Auron, who seems to think he knows everything and should tell everyone else what to do. Then there's Wakka, Tidus, and Lulu who aren't much better. Then there is the need to rotate everyone into a battle to get everyone experience. The final boss is really difficult, which, by itself, isn't a bad thing, but when it's coupled with having to go through getting those 10 crystals every time you want to try fighting it again, it gets really annoying. And that's just some of the reasons why, to me, FFX is, by far, the worst of the numbered series.