It is actually crazy how similar Cyrus is to Yugi - except for two things: he sucks at dueling, and he has brother issues. Actually, the sucks at dueling part might be entirely attributable to the brother issues, so really, it's only one major character difference.

I'm not entirely sure I'd characterize Jaden entirely as an expy of Joey - the upbeat attitude is similar, but that's honestly where the similarities end.

As for Kaiba, they split the main attributes of his character up into three and gave one to each of three rival-type characters, none of whom are as fabulous as the original. (Chazz comes close due to his hilarious richness and amazing catchphrase, but he mostly exists to have his haughtiness broken and to learn humility, which never happens to Kaiba and rightfully never should happen.)

I could ramble more, but I would hate myself for attempting to take GX seriously. |D