As NWest pointed out a few days ago, Romney and Obama actually agree on a lot when it comes to foreign policy. Both propose to leave Afghanistan by 2014. They're for drone strikes and having "all the options on the table" with Iran. Gitmo will stay open under either man. There are still some very important differences between them (to be discussed tomorrow in preparation for the debate), but there are undoubtedly broad areas of agreement. We are therefore denied a national debate on these issues where they agree.
A race between two candidates arguing similar, "moderate" positions is an unavoidable result of our plurality voting system. In a system where 50.1% wins and voters are conditioned to support the "lesser of two evils" by voting for the major candidate closer to their views, both candidates try to appeal to the middle, since they can, at least to a degree, count on the base/flanks to come along. Moving away from the middle cedes ground/voters to the opposition. If a candidate doesn't think 50.1% of voters agree with a position, he won't adopt it. In some areas, both sides will come to the conclusion that one particular position is the majority's view and adopt it, and we don't get to actually discuss it.
Differences in underlying philosophy, polling/focus group research and marketing strategy result in slightly different positions on issues, but both sides are essentially targeting the same voters. It's no surprise then that the range of ideas addressed by the candidates is narrow.
By having a system that pushes candidates to the middle, voters are denied a real chance to express their views, unless they are willing to "throw their votes away" by voting third party. Different voting systems like approval voting or instant runoff voting can accommodate a wider range of views, because they elicit more information from voters. Freeing voters to support more candidates means more candidates can be involved in the campaign, each bringing their own views. The presence of Jill Stein or Gary Johnson on Monday night would allow for exploration of the multitude of areas where Romney and Obama agree.