I have installed Cairo-Dock and Synapse so I have no use at all for the Unity dock and dash.
What more is there to Unity than that? The top bar with the indicator applets I guess. I don't mind it, but I wouldn't mind having another one either.
Can I disable the Unity dash and dock? Or do I need to use a different desktop manager? Which one? The lighter the better. Possibly one that does nothing but display applets in the corner of the screen.
If you are using Cairo-Dock 2.4.0 or later, a custom GNOME session for it should already be added to your login menu. You should be able to start into this session from the LightDM menu and get Cairo-Dock without Unity, no further fiddling necessary.
More detailed information for advanced configuration (if desired), as well as notes on common issues and how to get around them, can be found at this glx-dock.org post.
Here's how you disable unity.
http://linux-software-news-tutorials.blogspot.com/2011/10/ubuntu-1110-oneiric-remove-unity-and.html