One of the few potentially OK things to come out of the coalition government was the Lib Dem promise of electoral reform. I'm now pretty much against supporting it for a number of reasons.
1. The proposed system is not very good
AV is not a proportional system of electing representatives, but rather a preferential system. I, along with the Electoral Reform Society, back the Single Transferable Vote which has a precedent in the UK (its used in Northern Ireland and twice in elections in the 19th century).
2. If AV is adopted, it will stifle further reform
The political establishment will see this as 'finishing' constitutional reform. "You've got AV - what are you moaning about now". The Lib Dems probably hope that AV will be a stepping stone to a system of Proportional Representation. I disagree.
3. The Tories have spiked the referendum in order for fewer people to back it
In the bill for the referendum, they have also included legislation for reducing the number of MPs that conveniently favours the Tories. Consequently, lots of opposition MPs are not going to get behind it and support will be minimal. It can then be written off as 'Clegg's Idea'. Which neatly leads onto...
4. Its Nick Clegg's Coalition Dealbreaker
People hate him these days. Why would they want to help him get something he wants after letting so many people down?