Oh, c'mon, "New Guy's" comment was awesome, I know I enjoy the filter of unfiltered tobacco...or whatever the hell it was he was trying to say...
From what I can tell, in addition to cut (cigs are silk or shag cut, while pipe tobacco is more often coarse cut) and moisture levels (tobacco destined for the pipe has a much higher level of humidity than that destined for a cig), the biggest difference seems to be in the chemical additives (burning agents, anti-burning agents - yes, you will find both..., preservatives, etc) which are added to cigarette tobacco. I also seem to recall reading somewhere that most cigarette tobaccos are flue-cured, while I don't think one could say that "most" pipe tobaccos are cured using any one method.