Go look at the Mesa Matrix http://www.mesamatrix.net/ Nouveau supports more OpenGL features on their open source cards than AMD does.
Both Nvidia and AMD recent drivers (r600 and radeonsi) are 100% green on all OpenGL features that are currently officially supported (OpenGL 3.x)
They only have red spots for feature that are for OpenGL versions that aren't supported by mesa yet any way (OpenGL 4.x) - in other words, that's still getting worked on. And given the current pace of development, both cards will support all opengl 4.x feature with short time difference between each other.
(Note: the case of r300 is a bit different. It's an older card generation (The various Radeon 9600/9800/X) and actually lacks some features like unified shaders - unlike the nv50/nvc0/r600/radeonsi cards. So you'll never see 100% features support anyway. The hardware simply isn't there)
The problem aren't *features*. The problem is performance.
The only thing that's been holding the Nouveau cards back has been power management and even that's not a huge issue,
Except for the part that re-clocking is critical to get decent performance out of a card. And it doesn't work reliably yet. The usability is, according to current benchmark at phoronix, quite random.
That's not nouveau team's fault, though. Nvidia has started releasing documentation only very recently (and almost only about Tegra).
Without documentation Nouveau team has to reverse engineer almost everything, and that's not an easy task as shown by the actual realworld performance.
Nouveau has been also very rapid at making all features available to the newest generation of cards very quickly.
Except that real world test tend to show that the actual result will vary greatly between differnt cards.
I expect that by this time next year, they will have working OpenGL 4.2-4.3 support,
And probably the other drivers will have it too around the same time frame...
(You know, the whole point of Gallium being modular and parts being re-usable. Once Mesa starts supporting a feature for one card, getting the other to support is a lot easier: basically only upgrading the backend)
Whether Nvidia has posted meaningful contributions to the project or not is almost irrelevant. The reality is that open source Nvidia is coming and it's going to be great.
It *IS* relevant. Without any help from Nvidia, the work for Nouveau developer is much harder (as seen with the current problems regard re-clocking), and more bumpy accross the landscape of varied graphic cards.
As AMD provides documentations to the radeonsi/r600 developers (in addition to having some developer on their own payroll), it's much easy for them.
To the point that AMD considers the opensource driver as a valid alternative for older hardware whose support has been dropped in recent catalysts.