Lots of discussion about Gtk+ 3.0 lately, unfortunately there's no real discussion going on (heh).
There's the camp that is excited about API/ABI breakage and there's the camp that isn't so enthused.
I have to take the side of the "not so enthused", alongside Miguel de Icaza, Havoc Pennington and Morten Welinder: it's crazy insane to go breaking API/ABI for the sake of cleaning up the API without adding new functionality that could not be achieved by simply extending the current 2.x series.
There are definitely things that can be done to the current gtk code base without having to break the current API/ABI to achieve them, so in my humble opinion that's what should be done. Granted, eventually there will have to be a Gtk+ 3.0 which breaks API/ABI in order to add some new functionality that isn't possible to implement in 2.x, but that road can be crossed when we get there. No sense breaking it now if it isn't absolutely needed.
Even more worrying is the lack of promise to not break API/ABI again after 3.0. In fact, I've read on one of the mailing lists or somewhere that they plan on breaking API/ABI every 4-5 years after 3.0, ripping out any API's they marked as deprecated in the meantime. This is crazy. One-Flew-Over-the-Cukoo's-Nest crazy (or, if you prefer, I'm-Cukoo-For-Cocoa-Puffs crazy).
For large applications (Gnumeric, Evolution, Mozilla, Eclipse, etc), porting from 1.2 to 2.0 was a huge PITA but we all did it because (besides being young and stupid) 2.0 made a long-term commitment to maintain API/ABI compatibility going forward in addition to the fact that it added a ton of much-needed functionality (accessibility, UTF-8, model/view, and really good text layout w/ support for RTL and CJK).
If 2.0 was just 1.2 + sealed structs and the removal of some deprecated APIs, no one would have bothered with 2.0.
To respond to a point on Tim Janik's blog:
3.0 will ABI-incompatibly remove all deprecated and private APIs. Of course, the above described deprecation scheme only scales well if deprecated APIs are really removed from the code base at some point.
One reason that ISV's are typically on-board with Microsoft is that Microsoft has gone to great lengths to maintain backward compatibility with their old APIs. Joel Spolsky talks about it a bit in How Microsoft Lost the API War (specifically, the section The Two Forces at Microsoft).
The Windows testing team is huge and one of their most important responsibilities is guaranteeing that everyone can safely upgrade their operating system, no matter what applications they have installed, and those applications will continue to run, even if those applications do bad things or use undocumented functions or rely on buggy behavior that happens to be buggy in Windows n but is no longer buggy in Windows n+1.
I'm not going to argue that Gtk/GNOME should go to these extensive lengths to make sure buggy apps continue to work, but there's no reason that G*_DISABLE_DEPRECATED can't simply mean "we're not going to bugfix these anymore, what you see is what you get". You don't have to ever remove them, especially since it likely doesn't require much maintenance to keep them around. The port to Gtk+ 3.0 would require some work to make sure all the internals of things like GtkCList use accessor methods to anything that they use internally, but after that the job should be done and over with.
(ABI might not be a bit hard, but you could maintain API - thus meaning a simple recompile would work)
On the *giggle* *giggle* You should be using Mono/Gtk# *giggle* *giggle*-side, any application built against Gtk# 2.x will still work with Gtk# 3.x w/o any changes :-)
Oh, and Gtk# has DataBinding support (which, btw, does not require API/ABI breakage to Gtk+ nor sealed structs like some people seem to think on this morning's #gnome-hacker's discussion *cough* *cough*).