If John Kerry were elected instead of Bush then:
1) a faster response during Katrina might have happened (Bush did a pretty poor job let's face it)
2) The assault weapons ban would probably be re-enacted
3) The Stem-Cell funding bill would have been signed into law
4) John Roberts would not be Chief Justice of the U.S. Supreme Court, probably, either Judith Rogers, or perhaps Ruth Bader Ginsburg or Harry Edwards would be Chief Justice. Sandra Day O'Conner would probably not retire.
5) A quicker response would probably have been made to the middle east crisis, for better or for worse, depending on whether you are pro-israel
6) Conservatives will argue that terrorists would be more likely to launch an attack on U.S. soil if Kerry were President, however, these are just claims.
7) A more aggressive policy towards China would probably be enacted, I believe that because Kerry is pro-Union, and Bush is pro-Business, Kerry would force China to crack down more on the re-evaluation of its currency and the taking of jobs from the U.S., esp. in the areas of manufacturing.
I believe overall, the U.S. would domestically be slightly better off with Kerry as President. Internationally, it is debatable - it's hard to say how Kerry would carry out the war in Iraq. However, I believe that the middle east conflict would not have escalated to the degree that it has in the past few weeks.