Obtuse topic, maybe, but i was thinking about it after the thread yesterday when we talked about fewer/less than. I love the operators < and >, they are so easy to use.
Also, differentiation is quite majestic in it's simplicity yet it is so perfect. Possibly the coolest mathematical tool we've ever devised. Awesome.
I also enjoy proof by counter positive, or contradiction: proving something is true by proving that if it were false it'd not make any sense. :D Love it.