The whole shadow concept is definitely a cool idea, but seeing as it was all in Japanese for me, I didn't get any of that delightful Engrish. Ah well. As mentioned in the review, depending on your skill, Colosseum's story mode will be short to medium -- depending on how quickly you burn through it and if you bother capturing and "purifying" all the Shadow Pokemon. Apparently, Pokemon gender is random except for Espeon and Umbreon (and some others that are male only, like Hitmontop), so if you're the kind of Poke-fan who doesn't mind constantly resetting the game so that they can get female Pokemon, you can probably get a few more Pokemon through breeding and evolution.
Of course, if you have Leaf Green or Fire Red, you could always transfer a Ditto across and breed whatever the heck you want.

I've just managed to transfer a Meowth from Leaf Green over to my copy of Ruby without any problems.
As for the reason the boxes are so small and the like, yeah, it's due to the existance of Pokemon Box, the stupidest piece of software ever. The GB Emulation, Pokemon storage and extended Pokedex have ALL been moved over there, making the cooler features of Stadium absent in Colosseum. I was
really hoping that Nintendo would merge Box and Colosseum for the English versions, but with the announcement of Box in Europe, and the small box sizes already mentioned for the English version, it's obvious that Nintendo's planning to release Box as well and make us pay more money.