I'm just wondering if there is anything that is important to me that wont work in the new version of Splat.
 
Is there a list anywhere?  Doesn anyone know what add-ons wont work moving forward?
Any included plugins or instruments?

If they remain installed, they will work*.
 
*At least in this first CbB version. Beyond that, who can say?
 
All things being equal, why would they not work? You are licensed.

Melodyne 2 Studio does not.

All previously installed melodyne versions will work in CbB if they do in Platinum. There is no change in this area.

But what about when I get a new machine?
If I only install the new BandLab version, what products won't work?
I'm asking because I'm wondering how to maintain the best compatibility with old projects moving forward.

Installing BandLab Cakewalk will put the same amount of software as running the SONAR Platinum installer without any of the other plug-in and content installers.

It would be really cool if they could come up with a legacy installer to install all of the plugins which come with whatever anyone has bought up through platinum.  Just because it would be nice to have the latest versions of everything in one place.

If I only install the new BandLab version, what products won't work?

Then you do not get to use Melodyne and other 3rd party programs/plugins. This is now, Nut who knows what will happen 1 year from now.
As of now you need to keep Platinum installed to use those 

Because they are included for free with SPLAT that's the only reason. 

Melodyne Studio (3) works fine here. In fact, I've yet to find any plugin that fails in CbB*. Not surprising, given that it's built on the same code base as SONAR. Apparently, the IP agreement allows CbB to identify itself to plugins as SONAR, since all the proprietary third-party (e.g. TruePianos) plugins work, too.
 
*Well, there's Addictive Drums and Dimension Pro, but neither of those would run in SPlat, either. Some kind of license issue that I've never bothered to pursue.