Yeah, but look at Harry's upbringing.
He was brought up in the most Muggle household there is - with the Dursleys. The schools he went to were Muggle schools and his imagination was not allowed to flourish. I doubt he even was allowed to read a book besides the texts he read for school. Wouldn't you mind kinda be shut to new ideas if you were treated that way for 11 years?
Then he was plunged into an entirely different world - wizarding world, that is. There is a whole history of this world that he had no idea about, and is still learning about it along the way through his trials in the books. Even Ron, who can be a bit of an airhead at times, has an advantage over Harry because he was brought up in a wizarding household - magic is NORMAL to him, and he gets to see what's up in the wizarding world through the daily paper. Plus he has a whole community of wizards to talk to. If you were plopped into a totally different society, wouldn't you be a bit shell-shocked? It's like learning all your basic motor movements all over again - it takes TIME.
What I'm getting at is this: sure Harry doesn't seem to bright in these books, but he's playing a huge game of catch-up. And what normal teenage boy wants to spend hours reading newpapers and books and hanging out with professors to get a handle on the wizarding world?
I think we should give Harry a chance - he is maturing as a wizard, after all. And that whole Voldemort thing on top of learning wizardry - can you imagine how much stress that poor boy is under?
Ok. I'm done now