It is an interesting point from the OP though. Come a by-election, or even a general election come to that, how many people choose who to vote from based upon party allegiance, and how many based upon a judgement as to the candidates likely performance representing their constituency, regardless of party politics?
As to the result of this particular by-election - I think I was most surprised at the low turn out (48%) given all the controversy, media coverage, and current issues hogging the political media highlights at the moment - I would have expected a greater turnout myself.
Labour cannot assume that somehow they have been rehabilitated in the public percetion on the strength of this result. It also seems apparent that the conservative vote didn't collapse - rather, the majority of conservatives within the constituency decided to vote tactically.
Until and unless the Lib Dems and Conservatives formally create an alliance, to put up a coalition candidate would not be supported by the grass roots activists of each party, nor would you get much agreement for deciding which partys candidate should stand in which constituency.