Typically you want to check if there are certain mechanics that are patented, and then try to put your own spin on it.
I thought you may find this an interesting read — http://www.google.com/patents/US5662332
Now I believe this has expired because it was filled in 1995. But you still have to worry about any copywrite that they may still have relating to symbols and such. There have been may who have been successful in working around the patent in general. L5R (Legend of the Five Rings) got around it by calling it "bowing" a character and disregarding mana for gold cost. And you would only rotate the card slightly rather than the full 45°. Mind you they had to redesign the back of their cards because the Olympic Committee owns the rights to any five interlocking rings.
Hope that helps! :)