On May 5th the UK will be voting on (among other things) a referendum to use "Alternative Vote" instead of "First Past the Post" in electing members of Parliament.
The proposed system appears to be identical to that used in Australian Federal elections for the House of Representatives, with the difference that assigning additional preferences is explicitly optional. (In Australia votes missing full preferences are counted, but only by virtue of Electoral Commission practice and court rulings, and it's illegal to advocate partial preference voting - something Albert Langer went to prison over.)
AV is going to require very little change, contrary to some of the more ridiculous claims made by opponents. The ballot papers won't need to be changed at all - when I voted last April (as an Australian resident in the UK I get to vote in UK elections), I had to check with the electoral staff that I was only supposed to mark one box, as the ballot paper looked identical to an Australian one and my instinct was to write in a full set of preferences.
The counting will be a little bit more complicated, but not hugely so - this isn't anything like the proportional quota transfer system used in Australian senate elections! At Australian elections, only a handful of House seats remain uncertain after the first night's counting. The last election was unusual in not having a government decided immediately - and the UK managed that itself with FPTP, anyway.
Informing people about the change will involve some effort. Provided that "old style" votes (with say just an X marking one candidate) continue to be counted (as a first preference with no additional preferences), however, those who fail to understand the new system won't be disenfranchised. So if an education campaign fails to reach 100% of voters that's not a critical failing.
So what are the advantages of AV? It will drastically reduce the number of safe seats where sitting members are reelected without any effort. In the UK, members with 40-50% of the primary vote are pretty much shoe-ins. Even in Australia that there are a significant number of electorates where this happens (I've voted in both Bradfield, one of the safest Liberal seats, and Sydney, one of the safest Labour seats), but vastly fewer - a candidate needs 55% to 60% of the primary vote to feel really secure.
Practically, AV should help the small parties: people will be more likely to vote Green or UKIP if they can preference one of the major parties. On the other hand, these votes will mostly "come back" to the major parties as preferences anyway so that may not hurt them. The Greens in Australia have had only one real victory in the lower house (Melbourne at the last election) which is no better than they've managed in the UK (Brighton at the last election).
What will be interesting is the effect on the three-way dynamics of the major parties, which has no parallel in Australia (three-way Labour-Liberal-National contests are rare and rather different). The assumption is that AV will benefit the Liberal Democrats - and it's certainly something they've campaigned for - as a "middle" party who will collect preferences from both sides. On many social issues, however, they are more progressive than the other parties, so it's likely that BNP, UKIP etc preferences will pass them by, as will a good chunk of Tory and Labour votes. It also seems likely that the LibDem primary vote is in for a plastering at the next election, and AV won't help them if they're pushed to third place. (I predict a Labour landslide if the Coalition government lasts till 2015, since the economic outlook for the UK seems moderately dire.)
As an aside, I think Australia could really use a centrist "small l liberal" party. The Australian Democrats were half there, but the other half were left-wing progressives closer to the Greens. (And it was that divide which eventually crippled the party.)