One of the common refrains you'll hear about AV is that it's a system that allows extremism (see last post). The bogeyman of the BNP will be dangled.
However, it's actually FPTP that can deliver an extreme candidate, and less likely to be AV.
If you have a parliamentary seat with enough candidates, the vote can be so diluted across sensible candidates that a more extreme candidate can win. That candidate doesn't represent what most voters want, but because they happen to get the most of any individual candidate, they win.
Let me present a simple example using the real world: you, and a group of 9 friends are going to go to dinner together. 7 of you like a full choice of dinner including various dead animals. 3 of you are vegetarian. There's 1 really excellent vegetarian restaurant in town, and a choice of various dead animal restaurants (which also serve veggie food). You each vote for a restaurant, and the votes are tallied as:-
The Wholefood Gaia Vegetarian Restaurant: 3 votes
Gourmet Burger Kitchen: 2 votes
Harvester: 2 votes
Beefeater: 2 votes
Frankie and Benny's: 1 vote
So, under FPTP, they all go to the vegetarian restaurant, despite 7 of them really not wanting this, and in fact, it is probably their least favourite restaurant option. In practice, the Harvester, Beefeater or Gourmet Burger Kitchen people would pick one of those options and win. The vegetarians would have to settle for slightly less choice but overall, we'd have a more contented group whose choices had been more fairly represented.
And thing is, under FPTP which actually have seen something not too far from my example in the seat of Brighton Pavillion where the results were as follows:-
Lib Dem: 13.8%
So, 68.7% of people in Brighton didn't vote Green. Are they OK with that? Well, under FPTP we don't know. They might think that the Greens are bunch of loons who want to send us back to the middle ages. Many Labour and Lib Dem people might quite like them as they're going to expand the state, so while they'd prefer Labour or Lib Dem, they're not too unhappy.
Personally I think that even in somewhere as boho as Brighton, people of the Con/LD/Lab persuasion don't generally want the Greens. That in reality, most LD voters would pick Labour as a 2nd choice and many Green voters would pick Labour as a 2nd choice and you then have a Labour win. Maybe I'm wrong, but the only way to actually find out is with a system that gives us more information.