I got 3 extra points for time! HAHA I'm sure many others will do much better. I like that you went for a twist of a known classic; it's done well and the game runs smoothly. Most importantly, the core game mechanic -- a "click" to flip -- is very satisfying!
One possibility is to add an odd number of cards for certain categories. That way, after matching two opposites, you don't *have to match opposites again. You could potentially score a lower-scoring same-match!