Aeons:

They should have had Leviathan as a water aeon. He has been in every other FF game where summons are available, so why not this one!?

As for holy/dark, there wasn't really any holy or dark element in FFX, so it'd be pretty pointless. And I'd like it so much if they replaced Yojimbo with Odin! Just like Leviathan he has been in every other FF game where summons are available, and again, why not this one!?

Sidequests/Mini-Games:

I only managed to find Yojimbo on my own. Frankly, the mini-games were ALL poor, and yes, ESPECIALLY Blitzball. It's been said on numerous occasions but why was it an RPG? What is the point in an RPG inside an RPG? They should have done it as a sports game.

Two other things:

World Map:

It isn't more realistic not to have one. I mean, you go straight from one Location, to one completely different. That doesn't seem more realistic to me. Plus, finding secret locations is more fun than entering numbers on a keypad.

Characters as a whole:

Tidus was annoying, and acted tough when he was not skilled with a sword at all(look at the way he fell over in Zanarkand)

Yuna had a poor voice actor(Although I admit she didn't do too bad in the Animatrix) and plus her dialogue was just filled with lines like "yes" and "ok" and then the annoying "you know"

Wakka. OK, I can understand the whole racism thing, but why the hell did they feel the need to make him say ya CONSTANTLY!?

Lulu: In the Pre-Auron Times, she had a lot to do. She basically explained what everything was to Tidus. I need to replay FFX, so I can see whether or not she did anything after the arrival of Auron. But what annoyed me was at the start she was nasty to Tidus, then she just suddenly hero worships him(You've proved yourself as a guardian, maybe even the best)

Kimahri: What a waste! And that's all I need to say on that subject