World Tournament 2004 is geared towards those who want card-battling, and nothing else
Unlocking characters to battle for said packs is simple. Beat a group of characters enough times, and you graduate to a higher level, with more characters. By beating characters, you unlock different packs of cards, and by using (or excluding) certain cards, you unlock even more characters to battle.
The rules and gameplay are perfect. Where there might be certain contradictions, the game compensates for them with a clear, solid look on how the rules are applied and how the game works. If you want to test out a card combination, or see how well your deck works against an Exodia deck, I would recommend buying some variation of this game. It's like a real tournament, which is undoubtedly its goal.
There are only a few flaws. First, the gameplay seems slowed down, but you can easily fix this by holding down both shoulder buttons when choosing your opponents and holding them until the duel starts; doing this will speed up the battle. Second, the game doesn't tell you HOW to unlock some of its players, and you wouldn't know unless you read a FAQ or were fanatical about the series. Since beating the game involves beating everyone a set amount of times to unlock Yami Yugi, then beating him, not telling the player how to even get certain characters is quite a flaw.
There is ZERO story line, but it's not supposed to have one, is it? If you're looking to follow the Yu-Gi-Oh story arc, seek out another game. This is, however, the greatest Yu-Gi-Oh game pre-2005.