The new version of Quicktime (called X, for 10) has a suprising feature: no features! If you look for a “Preferences…” menu under the QuickTime menu you won’t find anyting. Consider for instance that you want to play a video as you open it – sounds logical, uh? – well, you have to work it a little bit…
Apparently, the only way to fine tune Quicktime settings is to operate via the command line terminal!
The most used: Auto play when opening, just write in the Terminal this line and hit “Enter”
defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1
If you prefer to have a GUI (a visual interface) for QuickTime settings, then you must consider the preference pane from this website.