First, it takes even more power away from the states. The Tenth Amendment isn't perfect, but at least be up front about wanting change it.

Second, it shifts even more power to population centers. Changing 1% of the vote of a million people is worth more than changing 10% of the vote of 10,000. Metropolitan areas might get a good look at every major candidate -- which is great for the national media, since they don't have to leave their air-conditioned offices to trudge to Iowa or New Hampshire -- but there is something to be said for getting a look at how candidates deal with people one-on-one and in small groups.

Third, it may extend state election debacles to the national level. Imagine Florida 2000 extended over the entire United States. At least with the current system, Oregon didn't have to recount her votes because of idiots on the other side of the country.

Fourth, it may also make national election fraud more attractive. Shifting the votes of a few percent on New York residents might not affect the electoral college at all, but it would have a very great impact if those votes were counted nationally.

Finally, if we are going to need a constitutional amendment anyway (and I think the national popular vote would be subject to constitutional challenges otherwise), it's not even the best method of ensuring that every vote counts. The best that can be said for it is that it throws less than half the votes away. If you want a *real* reform, take your reform wand and wave it over the Congress as well, and institute a national proxy vote. Every representative gets a voting power equal to the number of voters who vote for them, and they elect from their number a Speaker/Prime Minister/President/Whatever. Each voter's power is used in every vote, because his proxy is wielded by his representative. Leaders who get us involved in unnecessary wars can be voted out by the other representatives before a national election. With preference voting, you'd also have a definite order of succession (VP not needed), plus it would almost completely eliminate party politics.