Regarding "Legal Pitfalls," as long as you don't use the WORD "tapping," you're fine. The tapping mechanic is found in a huge variety of small, medium, and large CC Gs: A Game of Thrones, Legend of the Five Rings, Shadowfist, Over the Edge, The Spoils, etc. It's called bowing, depleting, cranking, and dozens of other names.
Considering Wizards didn't invent that mechanic (there are earlier citations of it) they seem aware that their patent is technically invalid. Unless you use the word "tap," they don't sue. Doing so would risk losing the ENTIRE patent.
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Additional note: Wizards has only tried to bring the hammer down twice: once on a game called Havic: the Bothering, which really WAS just dressed-up Magic disguised as a parody, and once on Pokemon, where millions and millions of dollars were at stake.
If someone is worried, apparently licensing the mechanic from Wot C is also deliberately cheap - a token offering of earth and water, so to speak.
I'm not very big on The Simpsons Trading Card Game, but I feel it's a game that's worth pointing out as it was one of the few released CCGs to be entirely non-combat (even that My Little Pony: Friendship Is Magic CCG has combat elements) and, due to its source material, is entirely humorous.
Granted, it didn't last very long, despite it being made by Wizards of the Coast, but it was well-liked by those who played it.