The area where Gnome excels today is in usability. All its preference panels are following the Gnome HIG, so there is great consistency throughout its included apps. A few things could always be designed a bit better, but because of the whole consistent and non-bloat look, it doesn't make the life of the user any bad.
The absolutely great thing about the HIG on Gnome is that it has won the hearts of all its maintainers, so when people are suggesting applications to become part of the main distribution of Gnome, they are instructed to HIG-ify their applications. It is absolutely imperative that developers read, understand and comply with the HIG as it is for the good of the platform in the long run. I like that. Example of a great-looking application that a non-core developer created with (obviously) lots of care: tsclient. This app is not HIG-compliant, but you can feel the extra thought its developer put upon.
While there is still no menu-editor for Gnome 2 (the Nautilus one doesn't work with Red Hat Linux 9 by default, and Nautilus is hardly the best way to handle this issue anyway), I must say that the "keep it simple" design of Gnome 2 is what won me over the time. I like simple and to the point UI designs. Bloatware is what I can't handle for more than 10 minutes. Gnome tries to not include more than 10-12 items per menu, as this is the maximum number users can handle "at a glance" (something that is important when considering a good UI). I believe this is the biggest difference with its rival, KDE. KDE tries to offer any possible and conceivable option and they run in the danger of clutter. Gnome instead tries to have good defaults, fewer options: less is more.
Despite the menu-editing limitation, Gnome 2 is quite flexible and extensible with the use of its taskbars. You can have many different panels and design your panel the way you want to. I wish someone adds an option to the tasklist to not display the name of the open application, but only its icon (and an ALT text when on-mouse-overing) in order to create a panel which "emulates" a dock a-la Mac OS X. Another feature I would like to see in its panel is the ability to have a "Quick Launch" area where all application launcher icons are the half-size of the Gnome menu icon size. This is important for both real screen estate (so I could put two icons vertically - e.g. 2x16x16 icons on a 32-pix panel) and clarity reasons (today it is difficult to distinguish the difference between an icon and the foot menu - they all have the same space between them and the same size with only a small arrow showing that the foot menu might be 'special').
Regarding look and feel, Gnome's default widget look is a very boring one, and maybe one of its weak points. Remember, "defaults matter", most users stay with the defaults, so these should be attractive and make sense. I would advocate for the creation of a brand new --modern and eye catching, but still comfortable-- widget set with a window manager theme where its buttons are well defined and quite large (similar to Bluecurve's size), as Gnome is at the marketing point that it should consider all kind of users, including newbies (in computers in general with... mouse movement problems ;) or simply people that require this form of accessibility.
Gnome 2 is much-much more stable than a year ago. I still get crashes with Yelp though sometimes (gtkhtml problems), while Nautilus' SMB VFS is just not as good as it should have been (and I am not aware of any work done to this recently). But overall, the system is way more stable than older versions and it seems to be getting better with each release.
Speed-wise the DE is not bad either, especially if compiled with the latest GCCs. A Pentium II 500 Mhz is a good machine to run Gnome 2 comfortably, but as always, the faster the better. GTK+ 2.x is much slower than GTK+ 1.x and this fact is especially visible on "heavy" apps like Abiword 1.9.x and Galeon 1.3.x. Another of my hopes is that gtkmm will follow more constant releases and somehow distribute their SDK with Gnome by default, so Gnome can easily attract some C++ developers.
There are two more issues with Gnome, but these are bound to 'naturally' get resolved as time goes by: Integration with the underlying OS and the File Open/Save dialog. Some will advocate that it is the distribution's job to offer tools that will manage the OS (e.g. networking setup panel), but I am not sure where the fine line really is so you can't say for sure that "this is an app that should come with the DE and this is an app that should come with the OS". If Gnome or any other DE can offer a solution in a consistent manner, then I am all for using it. Additionally, I don't like what some distros are doing, they create their own "system settings", "admin settings" and they also leave there Gnome's "preferences". It just doesn't feel right to have many preferences scattered all over the place in the menu. Gnome should find a way and create a framework for all preferences and with the use of easy to develop addons, distributions could easily add their tools without adding bloat, but only "publishing" their addons to the corresponding category that Gnome has pre-defined. Another integration issue is the easy mounting/unmounting of devices, I want to be able to see all my partitions on the context menus of "Disks" and not just what's available on fstab and I want mounting to be done automatically just by clicking the name of the partition in the submenu (the system to figure out by itself what kind of partition this is and mount it using the appropriate fs type). Yes, just like how BeOS gracefully does it. As for the file save/open dialog, this should be coming with GTK+ 2.4 in a few months, so hang on!
Also, I wouldn't mind seeing Gnome "emerging" in their distribution the changes Ximian did for their upcoming Ximian Desktop 2. Most of Ximian's work would require to be modified to better design principles (see my example here - no, not completely HIG-ified either) but these Ximian advancements have merit and importance and it would be a helpful hand to Gnome and its users. Other third party applications that would be great to join the club would be a HIG-ified version of Balsa or Evolution, Galeon or Epiphany, tsclient, Anjuta2, Bluefish, Gnumeric 2, AbiWord 2, gPDF, Gaim, Totem or a HIG-yfied Xine/mplayer gui, an internet downloader (like GetRight), a PDA/iPod sync suite, an iCal-like app, Gftp and yes, the GIMP itself.
Overall, I am optimistic about Gnome's development as a DE and I believe that with time this huge effort can accomplish an even cleaner and consistent interface throughout its application base for all its users. There is no doubt in mind, the only way is up.