She let's noone treat her like shit
What I love most about Summer is that she always stands up for herself. When she is mistreated, she gets angry as hell and makes sure the person who hurt her knows her anger. Summer's not someone you want to have around when she's angry at you. Which is actually the way it should be. So many people silently brood whenever they are unhappy with other people's behavious. Consequently they stay unhappy and the other person stays unaware of how much they hurt them and will therefore never change. I think Summer's way is much better: yell, cry, curse, protest, do whatever you need to do to express your anger and make sure the person who wronged you feels it.
She knows her worth
Yep, I guess you can say that Summer is quite arrogant. But she isn't arrongant about things she's not. She knows that she's a worthy person and all she asks for is to be treated accordingly. Which is truly the absolute minimum that we should all ask for.
She is always there for her friends
Despite her arrogance, Summer is never self-centered in a way which keeps her from being their for her friends (unlinke other characters on The O.C. like Seth Cohan). Whenever a friend needs her, she puts everything aside to be there for her. She goes through all the crap her best friend Marissa fabricates (which is A LOT of crap) but she stands at her side no matter what. She knows when something is wrong and she accepts her friends (especially Marissa) in whatever state they might currently be in.
She knows when something is more important than being right
Summer and Seth's relationship was one of my favourite things about the show when I watched it for the first time. I still think it's cute but I can also see now how fragile and at times unhealthy it is. They got through problem after problem and to be honest most of the drama involved is Seth's fault. From cheating over ignoring her to leaving her with nothing but a note and lying to her about major stuff - he does it all. And of course Summer makes all hell break lose every single time but in the end they always come back together. Everyone would understand if she couldn't forgive him but she does nevertheless because she knows that their love is more important than what ever catastrophe they went through even if she was right. This is real strength.
She always tries to improve herself
In the beginning Summer is portrayed as this this dumb, superficial party girl but she develops so much throughout the series. You can see it in the little things like her constantly listening to spirituality and self-help tapes. Or you can just think about how she studied all the Jewish rituals for Seth's grandma's visit (and even learned them by heart because Seth forgot to tell her that she can use the book) or dug deep into world politics to keep up with Zack's family. The biggest development is of course Summer becoming an earth- and animal-right protester in season 4. She might seem dumb on the first glance, but Summer's always willing (and suceeding) to improve herself.
She does everything with passion
This is actually one of the most important character traits for me: passion. It doesn't matter so much what you do, as long as you do it wholeheartedly as if your life depends on it. And Summer Roberts is an extremly passionate person. It doesn't matter if it's catching up on the latest celebrity news, freeing rabbits from a lab, helping her friends or planning a revenge: Summer Roberts does it all with passion.
She's an uplifting person
This might sound like a simple reason but how many people are out there (real and fictional) who are just downers, who are constantly loading their doubts and problems on you? The O.C. itself is full of these people (see: Marissa), so that it's a pure delight to watch someone like Summer who actually lifts you up. There should be more people like this, both in tv shows and in real life.