I keep hearing about potential cases of voter fraud in American elections, and most of the times I hear about that, vote machine hacking comes up. Voting machines also have the disadvantage of not leaving a physical, tangible trace, and I think this potentially complicates a possible recount, or analysis by a possible external entity. Also, since voting is not mandatory, it seems to me that there is no way for the common citizen of making sure that the displayed results actually match the votes that were cast. Furthermore (and I imagine that this will become less of a factor), some old people might struggle with electronic machines, even if they seem really simple, and you can't really put a supervisor in there to watch people use voting machines...
Now, paper ballots are simple, and in my experience they always worked. I live in an European country, and I always voted in paper ballots, from school elections to national elections. If the possibility of voting for write-in candidates exists, a blank space is even provided in the ballot. The case of Florida in the 2000 general election shouldn't even be a factor, as you can have a system where votes are crosses inside a square instead of the punching cards. Sure they probably take some more time to count, but I think nobody would mind sacrificing some hours in the name of transparency.
Frankly, there probably is some downside to this method, but I can't really see it. Why are paper ballots not used?