The Lumia 900 is similar to my prior android smartphone, the Motorola Atrix, in one respect: the front camera with only a fraction of the megapixels of the rear camera is way more foolproof, and tends to take better snapshots that the rear camera. For the life of me I cannot figure this out.
My 900 has the Tango update, and my quick judgment is there may some overall improvement with the automatic settings. The rear camera has always been capable of producing nice, contrasty shots under outdoor, well- lighted conditions. But indoors, unless there is very good light, or you are close enough to the subject that the flash will help, the results are not very pleasing. Bright areas tend wash out and also tend to produce a kind of effect that I would call a halo. Even outside, a bright portion of a scene exudes this halo, and it can sometimes not be corrected by changing the fiddling with the controls.
The front camera, on the other hand, does not have those problems. It does better in normal indoor light, and tends to produce a nice, snappy, contrasty snapshot. Turn the camera over and shoot with the rear camera, and you tend to get a blurry, washed out kind of effect. I am attaching an example of the same subject, one shot with the rear camera with no flash, default settings, and the other shot with the front camera. No post exposure editing has been done on either. Note the results.