Since 1.8 came out, I have always been playing on +100% brightness because it's easier to see things. One day I went back to my house which appeared to be fully lit and saw that a creeper spawned. No. Really.

That's when I realized something: the brightness slider adjusts how bright things look like, not how bright things really are, giving the illusion of brightness in a dim area and giving off a false sense of security.

Example: you are playing on brightness +100%, and there are some areas in your house which are at light level 7, the max brightness which spawns monsters. Since the brightness is 200% of what it really is (for those of you who can't do math, 100%+100%=200%), the area appears to be of brightness 14; the amount of light a torch gives off. Then, you go for a mining spree, and when you come back...

Brightness+ also makes things artificially bright - escpecially nightime, and well-lit areas (as seen with pictures). It also makes nighttime and dark areas less scary, detracting from the fun and difficulty. Remember those hectic nights in alpha where you were scrambling back to your base and every mob could see you but you couldn't see them? If you adjust the brightness all the way up, that will no longer be the case.