Realistic vs. Nonrealistic RPing is (I think) an interesting topic, and I'm glad it was brought up.

Realistic to me starts with having a character who is like a real person; they have strengths, weaknesses, etc. I try my best to make each of my characters have major flaws, and some strengths. For me it's more fun to make a flawed character and have them grow and improve their personalities over time rather than have them perfect and modeled right out of the gate. It opens up a lot more doors in RPing when your character can change.

As far as the eleven year olds having relationships, yeah, I think it's a little silly...but then again a ten year old friend of my family got a 'boyfriend' so ya know...
It's possible, but when I was eleven years old, it was pure puppy-love and being bestest buddies.

Now for the unrealistic. And I go straight to the characters with special abilities. My first character, Freya, knew at least three metamorphmagus' (sorry for the spelling), a vampire, a werewolf, and a character that nearly blew themself up.
It's fine to have them every once and a while, but when every-other person makes a similar character, the fun kind of extinguishes.
I'll admit that I'm guilty of making my character having special abilities, however they lessened over time and no longer exist (since they were a result of stress).

Perfect characters, as I've stated above, are unrealistic (and for me), very hard to RP with. And not every character can know about, or have experienced everything.
It's fun to find flaws in your own character as well as another person's. Most of us don't want a conversation along the lines of, "I can speak French." "So can I!", "All my classes are easy for me." "Me too!", "My parents were murdered when I was nine." "Mine too!", "I'm a world class pilot and flew around the world." "Me too!".

And I'll comment on something that Esme said, about how characters do and will clash with others.
Not every character can like each other, not everyone is perfect, and not everyone has the same views, even in the RP world.
Personally, I like a little clash every now and then in an RP. As long as it's not just dislike on the creator's side, then rivalries between characters can be followed through quite well and are actually really interesting.