As I noted on another thread, this is part and parcel for stuff made in China. Their QA is abominable. This is why you will see threads like this on virtually any product made there. Most of the time the quality is good enough, but they'll have a shift, or two, where thousands of units are made poorly of whatever widget they are making. In the US, most manufacturers are more stringent on the QA process during the actual build process itself. That QA process requires more inspectors, more review from line operators and technicians, and constant tweaking of procedures by Manufacturing Engineers and Test Engineers to avoid escapes. In China, they tend to just get the product to the point where it passes testing at the end (i.e. it turns on and appears to work), and lack a lot of the process improvements and checks required to get the quality of product out most people expect.
Apple implements a lot more quality checks which is why they tend to have better quality, but even they are not immune. So the moral of this story is that you should exchange your product for any defect you can't live with, but you should be leery of expecting perfection. If I were the OP, and the lighting was bad enough to really be a pain, I would go to the store and have them exchange, but make sure that whatever phone they give me, is satisfactory before leaving the store. I suspect that the bad lighting was a shift or two's worth of work on the production line during the initial run. Likely means a large number of the first run of phones had this problem.