I don't think that gnustep should be aiming to reimplement the entirety of OSX/cocoa. I think that it should aim for certain core technologies which are worth copying. That said, the video pipelining of OSX is definately worth copying. The fact that with a few quartz composer compositions and some glue code you can make an application like BoinxTV is pretty compelling proof that the video technology in OSX is second to none. I'm looking to replace a $30,000 video mixer using just a single Mac Pro tower with two video capture cards. It's the combination of hardware offloading of the video API (actually using all 8-12 CPU cores and the video cards), and the easy video pipelining that makes it so powerful. I haven't seen anything like it for Audio on OSX, (Thinking quartz composer but for audio) I know someone was working on something similar for 3D graphics on gnustep. I think the same about NEXT's DatabaseKit. I'd like to see that finished as well.
The rest of it, the eyecandy isn't really necessary. A lot of the eyecandy in OSX can be done by compiz, or similar window management software in linux anyway. When talking about system integration you're always going to have problems. OSX has the best written driver code at kernel level I've ever seen, drivers are 30-70% smaller on osx compared to Linux for Sound at least, and PCI setup is actually readable vs Linux which is a terrible rat's nest of unreadable code, and BSD falls between the two. That said filesystem support on OSX is ridiculous, Linux easily leads here with ZFS/XFS/BTRFS options.
If you'd like a comprehensive list of applications I'd like to see in gnustep I'm happy to provide them. Basically MPD and Cynthiune should have a baby that's a music database (iTunes), GSMplayer brought upto gnome-mplayer's level (I'm trying to do this myself.) System Preferences needs actual system configuration functions (Networking/X11/Sound) TalkSoup made stable, maybe just rolled into GAP, multitabbing on the terminal (seriously managing 10 terminal windows is annoying) SSH/SCP support added to FTP.app (because server permissions are annoying to configure when SSH can be leveraged) And of course a web browser. A stretch goal for me would be to refactor GWorkspace into two apps, one being GWorkspace, and another being either a finder rip-off or a new/better browser paradigm. I'd like there to be a few different options with file open/save styles and with file browser paradigms. GWorkspace could just be refactored to remember settings a bit better such as Browser vs Viewer mode etc.