It's pretty obvious that that the issue people care most about right now is the economy. And Obama and Romney have spent a lot of time talking about it--for the first two debates, they spent about 74 minutes on the economy and about 84 minutes on everything else (source).
The thing is, they just don't have that much control over the economy. I wish we could measure, or even estimate, the potential impact of a president with each issue. (I feel that given the breadth of political science, something like this should exists--I just haven't found it.) That way, people could judge candidates based on potential impact over all their issues of interest, instead of weighing a beast like the economy equally against things they have more control over, like healthcare and taxes.
An op-ed and video by Megan McArdle on presidents and the economy: