What effects could you get from playing too much video games

If the video game contains flashing lights or very quickly moving scenes, it can cause seizures and headaches, however this is very rare.
If you are sitting to close to your TV, you could damage your eyes, this can cause a slight blur to your vision, usually only a minimal amount, but sometimes a pair of glasses are required at around a prescription range (but dependant on how your eyes are effected) of -0.5 to -1.0.

Some people argue that video games helps with hand-eye coordination.
Others argue they can damage people's social life, and slow down the learning of life skills. If they are violent video games, they can cause a long term effect of being slightly less sensitive to death in the real world because they have been exposed to death in video games a lot. (This can be seen as a good think or a bad thing depending on personal opinion.)

A news article was written about a boy from the UK developing a blood clot in his legs from being sat down playing Xbox 360's "Halo Reach". This was because the boy wasn't moving his legs for suck a long time, the boy later dies in hospital.