I value the unique histories of each of my characters. Each one of them has played in entirely different ways: different guilds, different playstyles, different content in different eras of the game. While I'm proud of their achievements, I believe they are inextricably linked with the characters that achieved them. If I need to prove to a RBG leader that I've slaughtered hordes of the Horde, I'd appreciate a verified link to the character that did it -- but I have no desire to have that achievement linked to my little Cataclysm-era PvE bunny. It just doesn't fit.
I still feel odd that my characters share pets and mounts. I recognize that most people adore having everything available everywhere. To me, though, the things my characters have and don't have are part of what makes them unique, giving me different reasons to play (and not to play) each. It's perfectly OK not to have it all! Until recently, I'd been running about with my trademark Captured Firefly on characters such as my bankers that hadn't farmed it themselves. I've quit doing that now. It just didn't seem to fit.
To be sure, none of this makes me angry or resentful. I don't get pissy about a newer character running around with a title or mount he obviously didn't earn. I recognize how useful it is to have shared pets available for battling. And yes, like choosing not to summon my firefly, I realize that I can simply choose not take advantage of certain account-wide features. But after a bit of time under the new system, I find it still feels a little too glib. When characters become interchangeable, we lose our sense of investment in them.
Are you inseparable from your account-wide pets and mounts? Have shared achievements or titles helped you earn a spot in raids or join a guild? Have you started playing a different character based on gaining (or keeping) access to something you earned on another character? Have account-wide features made your characters feel more freely interchangeable -- and if so, is that a good thing?