In being a Relena lover, I will admit beforehand that most of these are biased and some purely my own opinion. It's what I see, at least.
I think that Relena's kindness is quite possibly the greatest strength she has. She reaches out to everyone, not just a select few she deems "worthy" enough for her attentions. She is forgiving and accepting, even toward those who would do her wrong (Lady Une, Mariemaia, Chris).
" Relena is so strong. I had lost sight of it — of the courage and strength it takes to really forgive someone. "
Despite all of the claims by many Gundam Wing fans, Relena is also very intelligent. She knows the workings of a politician's mind and she knows how to go about achieving peace without using violence. She isn't stupid for trying to stop Heero from fighting — she's brave. How can you call wanting to save people from dying stupid?
Relena pushes constantly. She's determined to put a stop to the fighting, to establish peace, to make her ideals reality. She's set on making Heero realize that she's an ally and willing to help him. She's determined to do good for the world, but also determined to bring about the change her own way.
" You're invading a sovereign nation! Drop your weapons! I will not tolerate your brutality! "
Sometimes, however, Relena's kindness toward everyone gets her into sticky situations. Although accepting Dorothy into Sanc Kingdom's school maybe helped her to somewhat become more reassured with her ideals, Dorothy also brought trouble for Relena. She ridiculed Relena under the guise of worshipping the ground she walked on, and I seriously doubt that Relena was oblivious to this. Dorothy also made the situation with the Romefeller Foundation a little bit more painful for Relena to accept, since she was right there to witness Relena's surrender.
I'd also have to say that Relena's thing for Heero (whatever it is) distracts her at times. She does bug him at the new schools he moves to and during his fight with Zechs in Antarctica, and Heero seems to be genuinely irritated at first. Eventually, though, it seems he's come to accept her behavior after he stays at the Sanc Kingdom with Quatre. Of course, if Relena hadn't been distracted by Heero enough to "chase" after him when he left, she might not have ended up in the political position she came to be in at the end of the series.