Couple things (this thread got everywhere awfully fast)
1. Mandatory Voting
I can count on one finger how many times I haven't voted since being able. That one time was because I felt all the parties were the worst of the worst options. I feel like being forced to vote in that Election would have been denying my "right" to vote. Yes, I could have spoiled my vote. I was actually considering writing my own name in...Anyway, mandatory voting bad.
2. On PR and the lack of Conservative Majorities.
One of the primary reasons to consider switching voting schema would be to increase voter turnout. Disenfranchising a large portion of the populous would have the opposite effect...should this actually happen. Although as other people have pointed out, the parties would likely change which would largely change how the votes are distributed.
3. On Schema
STV baby! Quite honestly, since the removal of the "party funding per vote thing", my vote hasn't amounted to anything. My vote hasn't resulted in electing a member of parliament, as such I feel that "I" don't have representation. Although to be honest in my riding, I would vote for my incumbent MP, but not his party (though I doubt under STV he would be ...bleh this could go on forever. next.
4. On how the parties would change. (hopefully for the better)
Given some form of more PR voting, I would hope that the party landscape would change significantly. More choice of parties would likely have the benefit of increasing voter turnout since the individual parties are more likely to find common ground with individual voters. Personally I would love to see a conservative party that had a much larger focus on the environment. The 'liberals' wouldn't have to pander as much to the far left, and the far left wouldn't have to pander to the center. And I think most importantly, the center right wouldn't have to pander to the far right, and as such would end up with a lot more votes. (The "anyone but" wouldn't be an issue)