Ed Kilgore notes one beneficial impact of John McCain's emergence as the likely Republican nominee:
It was most noticeable on the immigration issue, where the cramped defensiveness of past exchanges gave way to a wonkathon that mostly centered on the question of the extent to which illegal immigrants are depressing low-end wages (though Wolf Blitzer made every effort to drag the candidates back to the tedious and highly misleading question of drivers' licences). The simple reality is that John McCain's history on immigration reform largely takes the issue off the table in a general election contest. It could still play hell in down-ballot races, but unless McCain does a full-scale massive flip-flop, immigrant-bashing won't be a major feature of the presidential discussion.
This even has some influence on the Democratic primary, since Barack Obama's campaign seems to have been emboldened to take a clear position against turning DMV officers into a locus of immigration enforcement in a way that's helped him secure the endorsement of La Opinion and perhaps of Latino voters.
One very interesting question is how this will play out on the subject of climate change. In one possible universe, the fact that having John McCain as your opponent takes pure denialism off the table opens up a scenario where you have a debate between a timid strategy for tackling climate change and a bold strategy for doing the same. Thus, the political center of gravity shifts in a good direction. But in another possible universe, both sides vaguely pay lip service to the climate change either but neither really talks about it and the press just kind of writes this off as something on which McCain and Clinton/Obama basically agree.