Popularity affects fan interest which can boost (or hurt when popular players leave) attendance and therefore revenue. Perhaps merchandising revenue as well. Morale affects how well a player plays. For personality, see below.
I agree strongly with what Carplos says about work ethic. I find myself keying in on that in every draft and trade. A player with weak work ethic will probably never meet his potential, and I see AI passing on 5-star potential players and offering same in trades with red work ethic ratings all the time.
BTW, before you even think of asking, high blue numbers for Desire for Winning Team/Greed are bad. That's been asked before and confirmed.
