When I first thought about the possibility of it being a Regency-era fashion issue was that perhaps (don't laugh) it had something to do with the state of people's teeth at that time. I read somewhere that in Elizabethan times the most prized attribute that a woman could have was full set of teeth. Seriously, I think it is quite possible that Darcy considered smiling too much countrified and unsophisticated.

"The manner in which they spoke of the Meryton assembly was sufficiently characteristic. Bingley had never met with pleasanter people or prettier girls in his life; everybody had been most kind and attentive to him , ther had been no formality, no sitffness, he had soon felt acquainted with all the room; and as to Miss Bennet, he could not conceive an angel more beautiful. Darcy, on the contrary, had seen a collection of people in whom there was little beauty and no fashion, for none of whom he had felt the smallest interest, and from none received either attention or pleasure. Miss Bennet he acknowledged to be pretty, but she smiled too much."

It's interesting that he says he received no attention. Had he received it, would he have welcomed it? See, I have always considered Darcy a snob at this point. He looks down upon those around him and though he must acknowledge that Jane is beautiful, he must find a fault. She is not silly or sour, so she must be too sweet and countrified.