That's exactly what the MiniTube app does. It has a GUI of its own and within that you can search, download, add to play-list, thumbnails previews... and according to the developers MinuTube uses less GPU thus resulting better battery time for laptop users and interestingly it does not even use Flash player!.
It fully supports the 1080p HD videos as well. It is released under GNU GPL license and is available for Mac OSX and Windows as well. The developer is a KDE geek it seems :)... well KDE was born in Europe and they have a thing for that... so, it is written in Qt rather than GTK+ but integrates well with Gnome desktop
The latest version is 1.4 and in Ubuntu you can easily install it using a PPA channel. First open your Terminal and issue the below command.
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install minitube
That's it. For MS Windows and Mac OSX users, you can get the appropriate packages from here.