Pitivi is a great app, but I think the render dialogue could be made easier to use:

-Why do you have to click a button to set the file name and destination?
-Why do you have to click a button to choose the format?

Other information:
I'm attaching a mockup of a possible render dialogue.

-The name and destination fields appear right there, like your usual "save as" dialogue.
-The "container", "video output" and "audio output" are right there and can be changed without any hassle.
-Ideally, these should be filled with a "same as source" option in case the user just wants to join or trim a media file but doesn't want to change anything else.