Of course he was disappointed. He was great for the Rams, then got injured and had to spend many hours gradually coming back. Then when he felt he was at full strength or close to it - the system was changed, and he had to learn a new position that his skill set is not ideal for. He worked hard all season to learn the position, and did improve somewhat, and helped the team finally make the playoffs. Then out of nowhere he was traded for fairly low picks.
Players work very hard - Quinn certainly has - and a huge amount of their personal ego is tied into how they play, and often into their team. He was uprooted from the midwest out to California, spent a lot of time helping to win over a new set of fans, and then was shipped out. Fans are into their teams - but not as much as someone like Quinn was into the Rams. I don't blame him for being disappointed, and somehow I doubt any fans here were remotely as much into Quinn as a Ram as he was. People were disappointed - but it wasn't a huge upheaval in their lives.
Basically, so long as he's not making personal blasts, I don't care if Quinn indicated his disappointment, and I don't mind that an East coast small town boy felt overwhelmed by LA. He gave it a good faith effort, and then was shipped out as he was settling in.
I don't blame the Rams - it's business - but I don't blame Quinn for being hurt after all the effort he has put in to overcome a back injury, all the work he did to help the team, and then the work he put in to learn a new position, and the effort he made to help the Rams develop a new fan base in LA, even though it's a town he would never have voluntarily moved to. I understand fans not liking his comments - but they've gone through a fraction of what he has.