I'd like to make a further defense for Tidus. Yes, he is an annoying and whiny character. The thing is, that makes him a good character.

Tidus is a privileged youth. He grew up with the fame of his father, and later his own talents, which basically got him anything he wanted in society. He had tons of issues with his family, sure, but that's about it. What's more, he came from Zanarkand, which is already practically a utopia where everything is done for you and given for you (at least, in comparison to Spira).

This boy is then thrust into Spira, a world of despair and hostility, and everything begins to go wrong. His world shatters. Everything he was used to, everything he took for granted, even about the way people think and act, is gone. Not only is he a clueless newcomer who needs to be introduced to everything in the society, he is also someone from a completely different cultural mindset. He goes from being a penthouse dwelling celebrity to, essentially, a nobody commoner in a completely different world.

Not only is it necessary that he is introduced to the new world (which also serves to introduce it to the player), he has to adapt to it as well. And that adaptation would not be either easy or quick. Tidus's whining makes sense. The character is used to being privileged and entitled. He acts like a baby because he is, in essence, a baby. He never grew up, because he never had to, and, indeed, never really could. Zanarkand was such a bubble of perfection that growth and development in it were pretty much impossible. Remember what Auron said "Life outside the dream world may be harsh, but it is at least life". Tidus is having to respond and develop to this new world coming from a background which had, really, no hardship.

And Tidus does develop. It happens over the course of most of the game, sure, but even then, the game takes place over a relatively short period of time (seriously, a few months, tops, right?). His development, and even whining, while admittedly annoying, are still good characterization. It matches the characters experiences and personality at the time. Tidus's development is natural and steady, and his reaction to the bombshells dropped on him are all strong and well written.

Just because something is annoying or sounds ridiculous doesn't mean that it's bad characterization.