The eleventh commandment
Every roleplaying character must have an interest in other characters -- otherwise that character will be a failure. This is one of the differences between roleplaying and fiction; roleplaying environments are human social interactions first and storytelling opportunities second. If they fail to obey the laws of human social interaction, then the storytelling part will go absolutely nowhere. An author of a book or director of a movie can use special plot tricks and characterizations to force characters into lots of interesting situations even if they are egotistical, whiny, shy, or otherwise not very interested in other people around them (indeed the whole story might be how a given character learns how to correct their previously antisocial behavior). But in WoW, the lack of an author's absolute control over the environment means that this probably won't work for you; if you're antisocial, people will just ignore you -- end of story.
It's important to keep in mind that many roleplayers fall into the trap of trying to be the center of attention and entertain other people too much without even intending for their character to be antisocial in any way. In their imagination, their character is a wonderfully sociable person, and they don't fully realize what they're doing wrong. In fact, they may not even need to be in the spotlight -- they just don't know how to inspire other people to really roleplay and have fun at it.
Audience + Performers = Fellow Roleplayers
Especially at this stage of your relationship with other roleplayers, there's a big shift going on. The game is no longer about you entertaining other people as it was at the surface layer, instead it's all about establishing patterns of cooperation that work with individuals that you'd like to continue interacting with in the future. They're both your audience and your fellow performers, so everything you do is a kind of back and forth between you.
One important principle of this is that you always must work together with others, never against them. Even if what they say seems like terrible Mary-Sueish garbage, it's best to not come right out and say, "Dude, that's dumb." I once roleplayed a character who was a succubus in disguise as a blood elf, with the intention that she should be an entertaining spoof of other Mary Sues (she was terrible at seduction, for example, and she kept on letting her secret identity slip). As time went by, of course, I realized she had become too Mary-Sueish herself, but my guildmates never told me that! They just smiled and nodded at me all the time, and whispered to one another, "Nevermind her -- she's just a nutcase!" until I was ready to make a change in the character for my own reasons, not their imposition. When the time came to rework my character's story (since the original joke I had in mind had lost its novelty), the explanation was there already -- she was a nutcase! I added my own suggestion that she was now seeing a goblin psychotherapist of some sort and getting "special treatment" for her compulsive lying and overactive imagination. Doing so brought a lot of new things to this character and made her much more attuned to the group dynamics than she had been before. It also gave me a precious opportunity to learn from my mistakes.
Implications of interest in others
There are a number of possibilities that open up once you place the emphasis on other people's characters rather than your own. As several of our commenters pointed out last week, this enables you to have an open-minded and inquisitive attitude about other people, the game, and even your own character. It allows, even requires, that you refrain from excessive planning for your character on your own and instead work on it together with others once you get into actual roleplaying situations. Beyond the basic character concept and opening quirks you will use to strike up conversations or make your character memorable in some way, you needn't really set anything in stone except the general guideline that your character should be more interested in learning about others than promoting his own personal agenda. That's not to say he or she won't grow an agenda, just that you needn't decide what it is to begin with -- first talk to other people and see what sort of things people around you find interesting, and then fill that sort of thing in.
Suppose you want to create a mad scientist character -- this character should be on a constant quest for knowledge, always wondering if people around him have discovered anything he might find useful. In the process of playing, suppose you befriend into a group of roleplayers whose characters are devoted to eliminating the Scourge and defeating Arthas -- you'll want to connect your theme of mad science to that same goal rather than letting fantastical creations be your character's main goal in and of itself. If you've left your backstory open, you may even fill in something here: say that your character's old professor was killed and zombified by the Scourge, so now one of your main goals is to use your scientific creations to go and destroy the undead body that now imprisons his soul. If you joined a guild that focuses on protecting nature, you might make other choices -- likewise if you had joined a group that just likes sitting around telling stories and goofing off, you might just focus on making up silly scientific experiments your friends could participate in.
Whatever you choose for your character, remember that there's no pressure. Try something and see how it goes -- if you're really listening to people, you'll probably make choices they like. Whatever happens, it's a constant flow back and forth, where your ideas are weaving in and out of theirs, and improving all the time. Even if something goes wrong, don't worry -- if they're good roleplayers, they'll be happy to help you without condescension of any kind, because that's what true roleplaying is all about.