Yes, iTunes movies do not always play on QuickTime - some play, some don't. This is really annoying if you like to purchase the movie not simply watch it on many free internet sites.
I bought "Who's afraid of Wirginia Woolf" which is a very old movie and paid Apple GBP 9.99, later I found it on retrofilms.in free to watch.
The problem is that audio laggs behing video and movies on iTunes are just un-watchable that's why I installed QuickTime. This is a common problem - many posts on this on Apple's forum, many answers like changing your computer set-up - come on, I'm not a computer expert, I just like to watch a movie without spending time on re-adjusting my computer.
Apple is losing customers in a very simple way - when customers want to avoid hassle and unreliable film sites and they buy and download moveis from iTunes. Then, customers spend lots of time trying to browse through forum posts and installing unreliable software that is supposed to cure the problem. By browsing the web they finally find a site with their film being shown for free, which gets them even more upset with Apple... sure way to lose target customers. This is just waste of money and time purchasing movies on iTunes - I'm really upset with the service offered.