There are a couple of issues here. Firstly, that most people who bother thinking about it will realise that AV releases them from straightforward negative tactical voting - in other words, if you are a left-leaning LibDem supporter in a Lab/Con marginal, you don't have to sacrifice your LD support any more by voting Lab to keep the Tory out: you can vote LibDem if that's your preference, then give your second vote to Labour as a safeguard. And the same in any other scenario: suppose you're a conservative in a Lib/Lab marginal who thinks the yellow-blue thing is going well; you can vote for both parties in good conscience; if you're a Labour voter in a Lib/Lab marginal who despises the LD coalition "betrayal" and also hated the New Labour government and has always had to hold your nose to vote for New Labour even though you're far more to the left - you'll be able to vote for a full-on Socialist party and give your second vote to Labour as a back-up, thus being free to show your true allegiance without jeopardising the end result. In other words, all voters will be able to vote for whomsoever they'd like to win, then back that up with a second vote for the candidate they think is best placed to defeat the one they really don't want. This means that we will no longer be slaves to what happened in the last election in deciding how to vote this time; we will be able to start afresh each time. I think the vast majority of people in the country are really going to like that kind of liberating idea, as it'll sit really well with all our consciences. And it will give a much more accurate, genuine assessment of how the popular vote divides.
Secondly, this is one reason why Tories are historically against it: because they think that anyone who isn't a Tory will want to vote against the Tories and that the multi-decimation of their majority would leave them locked out of power forever. If the coalition starts working out, that might give many Tories on the ground pause to think that maybe that's not so much the case any more. So maybe a lot of grass-roots Tories will vote in favour of AV too.
Thirdly - and most importantly, perhaps - there is surely much more passion accruing from those who want AV than from those who are perfectly happy with FPTP. FPTP is a system that, even though you might agree with it, the concept of the defence of it is unlikely to inspire you to get out and vote for it.
I suppose what it comes down to is this: AV is not PR, but it still makes hung parliaments more likely. The only people who have something to lose in the long term from a continuing series of hung parliaments are the extreme wings of the Labour and Conservative parties, because only with the kind of solid majorities usually delivered by FPTP do they have a good possibility of any of their extreme policies being introduced. Even if they have to put up with the other party being in government now and again, at least they can enjoy the addictive high of righteous indignation while that's happening, and then eventually get back in themselves and swing policy back round to their side again for a few years, even if just a tiny bit - and a tiny chance is better than zero. (But these extreme policy shifts are not necessarily a good thing for the rest of us.)
Interested to hear what you all think on this.