One of the greatest feature on Longhorn is obviously WinFS... all the data (documents, images, video, music, contacts, email etc.) are grouped in a unique big catalogue and you can search this catalogue to find what you want.
The Find feature in Longhorn now appears like this:
Wonderful!! 
Now the way to search for a document is the same way to search for an email or for a telephone number but... what's the price of this power? This:
The WinFS.EXE process (alone) use 140Mb of your RAM and, if you consider what's the RAM usage of Longhorn without WinFS active (more than 500Mb, as you can see below)
you can see that the memory requirements are relevant (more than 700MB!!!!).
Longhorn eats a lot!! 
I hope that lots of memory usage improvements could be done from here to 2006 because meory usage must be well optimized.