First off all I know about gtk+ / qt problem, but it's not really that difficult to run KDE apps (applications) on GNOME and vice versa.
This idea is spinoff of all ideas about making Ubuntu just one distribution with option for choosing desktop interface gnome / kde /xfce.
Now if I decide to install gnome desktop or xfce desktop along my kde desktop I will have a lot of core apps like file manager or media player multiplied for every desktop one.
This would not be the case if the core apps would be the same for all of this desktops.
Here is list for proposed core apps:
terminal: this is only application that could prove difficult to use one in all of desktop managers.
File manager: probably nautilus, dolphin is not the best of file managers; i can't say how good Thor is. Or maybe some totally new application.
Media player: Mplayer I think it's very good and there is KMplayer for KDE, plus it has Firefox plug ins and it still lightweight for xfce.
CD burner: K3b many of gnome user prefer to use K3b, because its simply really good.
Browser: Firefox (no problems here; maybe no konqueror for kde; how many of us Kubuntu folks really use Konqueror?)
instant messenger: pidgin its really easy to make it work
(I can't get Kopete to use google talk)
Office suite: Open office
I don't remember any other core application, but you get the point.
I am aware that some of apps are specific and can't be same for all (like kate or kjots...). But this shouldn't be installed by default, but rather chosen by the user to install.
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