Is there no way that the sensor can tell if it is looking at a brighter or darker scene?
Yes, if it could use incadecent metering but as Mark stated, it could be combersome. The problem still remains of what it is photographing. It just ain't smart enough for that. Also as pointed out, if the camera could compare incandecent metering with it's TTL metering, it could possably much more accuratly measure exposure, but agian cumbersome unless something lie idea 2 comes about:
1) Digital cameras have the time and date. If GPS was added to that mix, the camera could figure out when the sun (and moon) rises and sets wherever it is. This info could be used as a basis for how much light (and what general WB temp) is available at any given moment given a clear sky. The photographer would still need to compensate as needed given the artistic intent, indoors/outdoors, and things like clouds/fog, etc.
2) Also, I'd like to see incandecent light meters that could transfer readings wirelessly to the camera. Perhaps over something like Bluetooth just so there is a transmission standard and no compatability issues. The camera could also transmit back to the metter and in effect, the metter could double as a remote controll for the camera.
3) The camera could have a means of selecting the general brightness meteringt. Simply press a button and scroll the wheel to select from a range of say five or six general luminosity presets. The camera would then meter for that 7% or 24% grey instead of the typical 18%. Combine that info with the area in the frame the AF locked on to. For example, if the subject is white (say a bird at mid-day) the camera would automatically try to keep highlights from blowing out in the portion of the image that the camera has focused on. Conversly, if the subject is black, the photog could select that setting and the camera would prevent the subject from being exposed too much.
These items wouldn't be too far of a streach to implement I would think. The debate lies with the practicality...