I'm new to Android. I understand that although the Nexus S is built by Samsung that it has a stock standard Android image (whatever that means) and so it considered a "pure" Google phone. If I were to buy any Android phone and install a clean Android image from source (I assume this is possible... I heard it is fully open sourced) then do I effectively get a "pure" Google experience a la the Nexus S? Compared to the Nexus S, would I be lacking anything (software-wise)?
In short, you can't just install a stock image from the Google source on any phone because to produce an image, you need all the drivers for the individual phone's hardware. If you can get hold of these, then it is possible to build a stock image but a lot of manufacturers don't release them.
For simpler solution (if you don't want a Nexus S or Nexus One) get a phone that can run Cyanogen and flash the Cyanogen image to the phone. This is pretty close to the pure Google experience, in that it doesn't have carrier or manufacturer modifications on it, but does have some Cyanogen extras included.