To simplify the interface kdenlive could take a tabbed interface, like the ribbon interface of Microsoft Office. Each card can be dedicated to a different function, such as: acquisition, editing, effects, authoring and export. Some pulsed, however, always remain visible, such as undo or the button to save.
What do you think about this idea ?

Doesn't Kdenlive have tabs already? And are tabs so much better than just do 'File > Render...'? Maybe you can make some sort of screenshot to point out the advantages...

I'm going to say that though it's a nice idea for other apps, video editing apps really need all the screen real estate they can get, and this idea would shove all the other panes about 100-300px. Not cool with me.

...and speaking of screen real estate (about which I totally agree with you), I also see that currently there is a duplication of the space destined to tab names in kdenlive's UI.

As you can see in the http://www.kdenlive.org/sites/default/files/theme.jpeg screenshot of version 0.7.7, the "Project Tree" tab title name appears both above the panel, and also below, in the tab itself.
I see this behavior in all new Qt/KDE apps, so perhaps is something related to Qt's handling of tabbed panels, but I would just eliminate all the top row containing the currently selected tab name and the two buttons, and put the two buttons in the space below the panel, to the right of the tabs.

That'd be, I think, much more efficient in terms of screen real estate use.
But, again, I don't know if this is something possible at all.

I've made a mockup to illustrate what I'm talking about. I'd appreciate your comments...
Thanks.

Really interesting, gab3d. The most important thing is that kdenlive continuing to be easy to use, so the users can appreciate this wonderful programm. Thanks a lot to everyone answered to me!