Hating other people (especially elders) for "preaching" is ignorance. The ignorance which has a hint of teenager-like mentality with all the "I hate adults!" "I know better than them!" "How they dare to tell me how to live?!". Doube ignorance, you see. If Wynne was contantly critisizing the PC or forcing them into doing something inappropriate I would agree. But don't be surprised if people frown at you for kicking puppies, stealing food from orphans or being in general selfish mean jerks cuz, well, jerks deserve it
Oh, but it's also how thread go fun 
Oh really.
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She constantly tells you you should sacrifice for the greater good and all that jazz. Which is all good and nice, except that the Warden probably knows that already. I've heard it a thousand times.
Wynne treats you like you're some inexperienced child who needs guidance, which can be fine to some players, grating to others. It's certainly funny when she speaks to a dwarf noble.
She's stuck in her idealistic views, sometimes unreasonable and blind. She tells you that your actions aren't proper for a Grey Warden if you bang Zevran. Apparently Wardens can't have sex. Someone better have informed Duncan.
And when confronted, you have no option of saying "Yes, I know what's at stake. No, I won't sacrifice the world to save my love, because, you know, we would all die. We could all die tomorrow for all I know. I'm content with killing darkspawn and having sex. There's a grand total of two wardens in the entire Ferelden, the responsibility of ending the Blight lays on me. I know that. Thanks. No, I don't want to return to the Circle tower because, hey, what about my duty as a Grey Warden we talked about earlier? No, I don't think your apprentice wants to come back either. He escaped from the tower. He's happy with the Dalish. Good talk."
I accept advice on how to live when it has a basis. But she doesn't offer wisdom, she offers banalities. Butting heads with people with different ideas can open your mind. Sten changed my point of view on some things. Hell, even Loghain. That only works when both sides listen though. I felt like Wynne wasn't listening at all, with few exceptions, not only with my Warden but with other companions and her former apprentice as well. Being old doesn't automatically make you wise.
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After all the babbling, I'd like to point out that I don't hate Wynne. I think she's fairly smart and compassionate. She usually approves my Warden's choices. Her personality simply grates on me sometimes.