I think there’s one addition to be made to clairobscur’s list of non-voters. In countries with a high level of immigration (such as most of Western Europe, for example) enclaves of ethnic minorities develop, generally in inner cities. At least in the UK, but I suspect it’s a general rule, there is a very low participation in elections among these groups, for various reasons - the language barrier, lack of integration and the fact that no-one is addressing their specific concerns.
Under a compulsory voting system, these concerns would have to be addressed. Ethnic minorities are a large enough segment of the population to sway a marginal seat (in a first past the post system) or to gain seats under PR. As a consequence, politicians would have something to gain from representing the interests of this group and would actively pursue votes - not least by printing election materials in different languages, but also by producing the policies that will win these votes. As has been pointed out, a large segment of the current voting population (and non-voting population) have little interest in which of two main parties gets power, but it could make a great deal of difference to those whose needs are different.
Essentially my point is that in U-countries government acts in the interests of the voters, and in A-countries it acts in the interests of all its citizens.