In a nutshell it says... :
No Delay Designer
No Space Designer
No vintage instruments
No support for Nodes
No TDM support
No Jam Packs
No SoundTrack Pro
I don't usually go for the light version of such applications, and I had assumed that Express wasn't even an option - until I read that comparison chart. Though I'll definitely be upgrading to the Pro version sometime in the future, I'm still very impressed to see how much stuff is included in Express.
Incidentally, this isn't likely to happen, but considering that Express perfectly suits my needs as is (no need for the 3 aforementioned features), I really wish we could buy the additional stuff on an individual basis, as needed. As an example, the only thing I really would need from the Studio Bundle would be Waveburner. I'll never use Soundstage and the rest of that stuff.
Same w/ plug-ins and additional content. There's like 60% of that stuff I honestly have no use for.
That's one of the reasons I opted for Express in the first place.