I hate to throw around the word "change" after hearing it incessantly from the candidates this past couple of weeks, but it is undoubtedly the best word to describe what we should be facing and thinking about as we go to the polls. The issues that have crept up in the last eight years, but not gotten their due or appropriate response...healthcare for one. The issue of healthcare in this country has been inching closer and closer to catastrophe/climax/showdown...whichever one you think is more appropriate. Healthcare needs to be overhauled, from scratch in my opinion, to meet the needs of changing demographics and changing times.
Whether Obama or Clinton end up winning the democratic nomination, an issue that will strike a chord is discrimination and intolerance. A black man and a woman, they'll be much harder to pressed to uphold their views on minority and equal rights, which will eventually mean some kind of resolution on the gay marriage debate, abortion, and maybe gays in the military.
I don't think it'll be a big issue before the election (unless people start dying directly from it), but global warming, energy sources, and big oil will definitely become prominent in the news hopefully in policies during the next presidential term.
Obviously Iraq will also be a big issue, but Iran and/or Pakistan may become contentious too if they stir up trouble (or if Bush stirs it for them).