Agree that there are a number of issues here that the currently most vocal group in favour of PR haven't necessarily isolated.
First, there's the fact that the LDs projection of seats was about 90 with 29% of the vote, whereas Labour would get 240 off the same percentage. I absolutely agree that is not fair. PR would fix this, but it would also have given the BNP about 8% of however many seats the LDs ended up with. So then you get into open/closed list geekery and spending 5 mins on the electoral reform society's website should disabuse you of the idea that PR is worth doing.
Second, there's LD supporters' disappointment that they actually didn't make any gains this time. They lost seats, but maintained their 23% share across the country. You can make a guess why but you can never prove it - one reason might be that people got to the polling booth and realise the number three guy is just not going to make it, so to make sure of kicking out #1 you have to vote for #2. Or they decided the policies didn't add up in the end.
Third, and I haven't heard much from the LDs on this, is the boundary change/constituency link/size of government issue. I personally want to be voting for an MP who is representing our part of the country in Parliament. Where possible, obviously, and I realise that constituency business tails off if you are a minister. I think there could easily be 20% fewer MPs. And I don't see why boundaries can't be a lot more flexible in re-sizing according to numbers on the electoral roll at a point frozen at say three months before. Large numbers of new registrations close to an election have got to be on the dodgy meter, and 'transfers in' ie people moving house just before an election are balaced with a removal on a register elsewhere and probably quite few in number.