Top 10 US cities by population ( http://en.wikipedia.org/wiki/List_of_United_States_cities_by... ):
The 10 largest MSAs contain over 80 million people. That's already over a quarter of the country -- more than the 50 largest cities put together. Taking it out to the top 20 MSAs brings that up to almost 117 million. The top 50 MSAs cover more than half the country's population.
Edit: restricting it down to "urban areas" instead of MSAs brings the numbers down a bit, but the top 10 and 20 still make up a huge amount of it ( http://en.wikipedia.org/wiki/List_of_United_States_urban_are... 2010 instead of 2011 numbers here ):
Top 10: 73.4M
Top 20: 101.8M
Top 50: 143.1M
It's hard for me to imagine the average resident of, say, Jasper, GA (population ~2,000, median income ~$30,000) caring one way or another that you had a rally in Atlanta— but their votes still count.
The more general point being that the dynamics of campaigning would be fundamentally different, such that none of the lines that are currently drawn will matter (and of course that's good thing).
Imagine a replay of 2012 without the electoral college. Obama would still hit the urbans and some of the progressive rich enclaves. Romney would hit all the suburban, Wall St. and yacht clubs.
No one would bother with the rurals except for some photo ops. Same as before.