Just want to ask about the full screen icon that used to be on the play bar but seems to have disappeared since upgrade to mountain lion, any way to get it back?

Since upgrading to mountain lion I've noticed the full screen icon which used to be on the playback bar in quicktime has gone, I know you can go to the menus to make it full screen but if you're watching something online like with apple trailers I can't seem to make them fullscreen anymore, is it me or have they just taken this option away now?

The answer depends on how you are actually viewing the trailers. If you are using the download mode, then "No, it is not just you." The QT X v10.2 player "Full Screen" button was replaced with a "Share" button popup. However, if you are viewing the trailers in the browser mode, then, "No, it is you having a problem."

In any case, if your problem is one of a desire for consistrency between the two viewing modes, then I suggest you switch to use of the keyboard shortcut to switch between viewing modes. By this, I am referring to the "Command-F" combination to turn on "Full Screen" viewing and the "ESC" key to turn it off. These key chortcuts are consistent between both the QT X v10.0, 10.1, and 10.2 players and the current browser viewing emplementation unlike "Command-F" which is only a one-way switch for browser.

I've been playing the trailers in my browser and the full screen option has gone for me,

As already stated, the icon was replaced by the "Share" pop-up menu icon in the same location.

and the command-f keys don't work either when I'm playing in a browser and it's truly only been since I upgraded to mountain lion.

Strange! It works for me. Here is a link to a screen recording of me wayching a trailer in Safari and later watching a trailer download. In both cases the trailer could be expsnded for full screen viewing and then reduced back to the size of the original 720p player display screen. If you can't do this then you may have a player problem also.

http://downloads.walker4.me/downloads/Temporary_files/ScreenFlow_6.mov

Why did apple make these changes when they worked perfectly the old way, I mean if it ain't broke don't fix it!

Only Apple whould know for sure but my guess would be that other users may have requested the "Share" pop-up more. Makes no difference to me because I normally use the key shortcuts anyway.