It never really clicked with me as to where Tidus learnt how to fight. He was given Jecht's Longsword by Auron and after initially fumbling with it for about three seconds, he's suddenly a seasoned pro. Cloud, Squall and Zidane by comparison seem to be much more 'qualified' to save the world (mercenary/ex-SOLDIER, military school graduate, (SPOILER)alien thief, respectively)

I also have another theory as to why people seem to dislike Tidus more than the protagonists of other FF games. It all boils down to the fact that no one says his name and as a result, we all feel somewhat distanced from him. In the days before voice-acting, it was a non-issue. In FFX-2 and FFXII, you couldn't rename Yuna and Vaan. Maybe if Square never gave the player the option to rename Tidus and have him mentioned and participate in dialogue more naturally, we'd all like him more.

And his pants are uneven.