The lack of a world map, the fact that, as mentioned earlier, it is just one line you walk along. The story is very basic consisting of "Destroy Sin! Stop Seymour! Destroy Sin!" with the occasional "omg, he's dead too!" and from practically the very start you know that this game will conclude with defeating Sin. Other FF's that I've played at least don't introduce the enemy until you have some experience under your belt with a storyline that builds to the realisation of what the true story is.
There's also the characters fulfilling various cliche stereotypes, but that's common I guess...
They're the flaws I see in FFX at least, despite FFX being one of my favourites (joint most favourite with IX). Could probably think up more but can't be arsed, plus its not this threads topic.
Anyway, if I have a PS3 by the time of its release, which is doubtful, I'd buy XIII first. I'm eager for a female lead in beautiful graphics, and from what I've heard about Versus doesn't appeal to me.