Nouveau

nouveau is the open source driver for nVidia graphics cards.

Limitations
Being an open source project running on what is largely proprietary hardware, there are some limitations inherrent to Nouveau that are not. That said, Nouveau is constantly evolving, so these limitations are subject to change over time. Some of the current issues are stated below:
 * Performance (especially 3D performance) on the same card/chipset may be significantly worse than using nVidia/nvidia-drivers.
 * Attempts to run at higher refresh rates (i.e. above 60 Hz) may fail.
 * Using advanced features from various Nvidia cards (e.g. multiple-display capabilities of Quadro cards without SLI) may not function correctly.

Kernel
You need to activate the following kernel options:

xorg.conf
The X server is designed to work out-of-the-box, with no need to manually edit X.Org's configuration files. It should detect and configure devices such as displays, keyboards, and mice.

However, the main configuration file of the X server is the xorg.conf.

Multi Monitor Problems
If you're having problems with freezing with possible EDID probes when using dual monitors/displays, try adding "video=VGA-1:e" or "video=VGA-1:d" (enable or disable respectively) to your grub.cfg/grub2.cfg command line options, substituting the name of your monitor listed within dmesg or /var/log/Xorg.0.log. (ie. CRT-1, VGA-1, LVDS-1, DVI-1, ...)

I have a Dell Inspiron 8100 laptop with a connected external display connected via the laptop's external VGA port. My current command line is: "video=LVDS-1:1280x1024@60 video=VGA-1:1280x1024@60". Think the EDID (DRM) probe on the external VGA connected display is still causing freezing during nouveau/drm load, but at least it's usable if the display is connected after grub is loaded and prior to the nouveau/drm modules loading. And with the prior mentioned command line LVDS/VGA resolutions, the displays are cloned and centered with somewhat correct resolutions for cloned displays.

NVIDIA Optimus
If you are using a laptop with NVIDIA's Optimus technology (usually found in laptops built in 2010 and later) then everything will be passed through your integrated graphics controller (usually Intel) before it gets to your discreet video card (NVIDIA), which can cause a lot of otherwise unexplained problems when enabled. There are a few ways to handle this, but the simplest is to disable Optimus through your BIOS (normally accessed by pressing F12 during boot). This will result in increased power consumption (decreased battery life, increased running temperature), as your NVIDIA card will now be handling all of the work all of the time.

I'd like to post something here about how to get Optimus working properly, but I haven't figured it out enough to do so. I would suggest looking into the kernel option "config_vga_switcheroo" and the Bumblebee project to start.

External resources

 * Nouveau Video Acceleration