1) Tim claims that compulsory voting will make elections less of a beauty contest. In my mind, this couldn't be further from the truth. People who are not interested in politics, and thus not voting in elections, are even more likely to be swayed by appearances etc.
2) Another impression I got from Tim's column was that he assumes politician's chase votes (which is true), but voters are completely self-interested. The non-voters can still be represented by voters who care about them.
3) The sole reason one would wish for compulsory voting is to increase voter turnout. I imagine most of the non-voters feel that their vote is worthless, or that their vote doesn't make a difference. There is no reason why, under compulsory voting, their vote would make any more difference. Firstly, unless non-voters are a completely homogeneous group, no single party would gain massively. If participation in elections is completely random, then the election results are completely unbiased. Of course, non-voters are probably not a random subset of the population, but I also doubt they are as homogeneous as Tim thinks.
4) Another reason why people feel their vote is useless is because, sadly, it often is. In many safe constituencies, I doubt compulsory voting would make a difference to who won the seat (especially if the non-voters are heterogeneous). A much better way to fix the problem would be to alter the electoral process such as by introducing the alternative vote.
5) I don't personally think the state should be able to force people to vote if they don't want to.
These are just some thoughts on compulsory voting and why I think it is wrong. Comments are appreciated.
Finally a quote I saw (from Chris Snowdon of the IEA): "The ability to vote sets up apart from the animals. The ability to not vote sets us apart from the Australians."