My keyboard backlight is fairly dim. You can definitely see a bit of light through the keys, so it's functional, but it's uniformly more dim than it was when I first bought it and definitely more dim than my 13" 2011 MBP keyboard backlight. Any ideas?
White LEDs, as used in the keyboard backlights in apple laptops, do dim over time.
White LEDs use a blue LED, and an phosphor which is excited by the blue light, and re-radiates the energy in the orange-yellow spectrum. Unfortunately the phosphor does decay over time, reducing the efficiency of the LEDs.
However, the lifetime of modern LEDs is typically something in the range of ~50,000-100,000 hours to 50% brightness, so it's likely that there is some other mechanism in play for your situation. There are a number of external factors that can effect LED brightness, from poor-quality phosphors to overdriving of the LED die.
Can you estimate the brightness difference? If it's only a few percent different, it may just be normal aging, or lot-to-lot LED variance.
Anyways, if it bothers you, it may be covered under the apple-care warranty. It might be useful to take it to an apple-store and have them look it over.
Here is an interesting white-paper on LED life expectancy and ageing effects.