I can't find this information in online help so I'm not sure if it's true, but the post #9 in the thread below says that plugin upsampling only works when doing a non-real-time bounce. So, if the information is true and if your project is recorded at 44.1kHz/48kHz, fast bounce should probably make the project sound more 'open and lively' (if plugins that benefit from the upsampling feature are used in the project). 
http://forum.cakewalk.com/The-Foxboro-PlugIn-Upsampling-Thread-m3247497.aspx
 

Some plugins don't work properly with Fast Bounce. NI Battery is one example. I tend to use it most projects, so I got out of the habit of using Fast Bounce a long time ago.

Bouncing is definitely a mysterious thing in Sonar.  I spent a lot of time trying to figure out differences, why and how --- dramatic changes can occur from the "Source Category" choice and from the "Source Hardware Outputs" choice.   The 64-bit engine setting also may be an issue (I always leave it checked).  
 
Another big reason that changes can occur has to do with controller lookback -- if you are attempting to bounce just a section of a project, there's a strong likelihood that your mix will be quite different than if you bounce the whole thing. 
 
Another thing that popped out of previous discussions had nothing to do with Sonar -- if you change the bit depth (accidentally or intentionally), that can change the sound too.   Probably obvious to many, but it caught me when using a default "save-as" that had 16bit set -- this was in SoundForge, working with audio from the Sonar bounces.
 
There was at least one long thread about this, maybe 6 months ago.  Probably many threads over the years. 
 

Bit depth would make a massive difference IF YOU DIDN'T DITHER.
 
If you DID dither then there should be no audible degradation, maybe with the exception of extremely long reverb tails at a very, very low level.

icontakt
I can't find this information in online help so I'm not sure if it's true, but the post #9 in the thread below says that plugin upsampling only works when doing a non-real-time bounce. So, if the information is true and if your project is recorded at 44.1kHz/48kHz, fast bounce should probably make the project sound more 'open and lively' (if plugins that benefit from the upsampling feature are used in the project). 
http://forum.cakewalk.com/The-Foxboro-PlugIn-Upsampling-Thread-m3247497.aspx

Good find. I'd missed that post.
 
I'm wondering if Craig mis-spoke, though, and meant to say the oversampling is disabled during an audible bounce. There'd be no need to avoid the overhead for a slow bounce unless you're simultaneously playing back the mix while bouncing.
 

As I'm trying to figure out the sends on non-solo'd tracks, I remember another feature of bounce:
 
THE ROUTING OF THE DESTINATION TRACK MAKES A DIFFERENCE!!    
 
Not at all clear why this should be, but it is.    So:  if you use "New Track" for your bounce, that will have some kind of default routing.   If you create an audio track, and set its output routing, and then use that as the destination for your bounce, then there will quite probably be a difference in the sound.