There was a great article published yesterday on WPMU.org called “9 WordPress Myths That Might Surprise You.” Good stuff and common questions that we get all the time. All of the following are myths. You can read more in the article. Enjoy! 
Too many plugins slow your siteInactive plugins slow your siteInactive themes slow your siteToo many revisions slow your siteKeeping a lot of trash or not emptying the trash slows your siteWordPress is for blogsWordPress is unprofessionalWordPress websites are low qualityWordPress is insecure
These most certainly are all myths as web.unc.edu has proven time and again. Take a look around to see some of the great things that are being done on campus with wordpress.