turtle.getLightLevel() could return the light level.

This could allow turtles to automatically determine which areas are dark to light areas, and also could be used to time things to sunrise/sunset (although this can be determined with the system time also), as well as mob spawners.

My, bad I saw some others here and didn't know there was a better place for them.

Personally I feel this fits better as a peripheral rather in the base turtle API. Ignoring turtle.detect(), turtles are pretty much blind. I'm pretty sure Plethora's daylight detector provides this functionality, and I'm sure there are other mods too.

I feel compelled to mention that dan has stated he doesn't want suggestions here. It's in a few other closed suggestions as well, this was just the first I could find.

To expand on @KingofGamesYami's comment, suggestions generally go on the Suggestions sub-forum.