Well, there's a technical side of it—and I'm not a computer scientist and there are other people better qualified to talk about that.
The major issue is the lack of transparency in the process. Things disappear into cyberspace and nobody's entirely sure what happens and you can't recreate a paper trail the way you can with a paper ballot. That's a significant issue.
There's the issue of identity verification. Are the people casting the ballot actually the people who are supposed to cast it? How do you prevent ballots from being sold or those identities from being traded off? There are all kinds of issues around that, which I think are fairly significant technical challenges.