1) How do you keep the game interesting so that people don't abandon it after a the novelty wears off?

Tournaments and ladders, with prizes being gil, gear, more cards, etc.
Frequent updates, and perhaps integration with other content.

2) Create a reward system for a card/mini game that a) cannot be exploited by players, and b) doesn't run afoul of national gambling laws*?
Constant revision. I believe its Magic: The Gathering which basically "evolves" every expansion, but still gives relevance to old cards, without physically changing their stats, or simply outright banning cards. On the other hand, the Yu-Gi-Oh TCG lacks any form of this, and old cards are simply casted aside as useless. On the other hand, the latter's method provides a constant demand for new cards.

Do countries really have gambling laws that cover online games? I know Korea has laws in place to prevent RMT or something like that, but as far as gambling over a game?