Bad eye strain from staring at the screen too long, like when playing a game of Civilization straight through without breaks. Back pain from being so engrossed in the game that I won't move from my chair. When I get up my back is stiff and I need to stretch.
I had wrist pain when I was using fightsticks on my PS3 after not having used an arcade stick for about 10 years after arcades disappeared. Took me a while to get adjusted to the fightstick and holding a balltop stick and learning finger placement again.