Somebody mentioned the Aeon leveling aspect. That was a good touch, but I agree, not terribly useful. Really once you get Bahamut, Anima, and the Sisters, they attack at such a high level it wouldn't have mattered if you leveled up any of their strengths or magic anyway.

The sphere grid I thought was actually really fun. It's not really about trying to get everybody everything. That's hardly necessary anyway. But you have a few characters like Kimarhi and Rikku that you have to make some decisions with as to how you want to use them. I recommend sending both of them down either Wakka or Tidus' path early. You could send them down Lulu's path, but it takes the entire game for you to get either of their magical abilities on par with hers. But if you do, and you get all 3 of them double cast at the end, then they're really really strong. BUT, you won't need it. It's much more useful to have them as warriors throughout the bulk of the game and with all of the other attacks you have, the final battles are easy anyway.