I prefer usually challenging questions, while I may spend some time on easier ones trying to explain something that used to be difficult for me. This is not the most rewarding way to get reputation points, but this is how I like to answer stack overflow and server fault questions, when possible.
My preference goes to algorithms, and C, a language that I've liked and known for a long time, while I like also to visit server fault and its many interesting and challenging system, DB, nginx, ... questions and answers.
Linux has been my working environment for almost 10 years. And a (few) Mac for leisure.
As of now, I never gave a negative rating. This is the best way I found to move away the temptation to abuse the down voting ability.
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