Whereas filing your taxes online is a one way transaction between you and the government, voting is one big symmetrical transaction between every member of the electorate. This makes it really tricky from a security perspective.
The reason that internet voting is worse than postal voting is the slowness of messing around with paper, the same reason which people tout as a positive. You cant change a million postal votes in a under millisecond from thousands of miles away. Think of the postal service as hash iteration.
For (lots) more info, see the following Google tech talk, "Electronic and Internet Voting (The Threat of Internet Voting in Public Elections)" http://www.youtube.com/watch?v=_GjmRwfkRXY