This a summary of new things which have been added to EndNote X.

View the contents of up to 8 different fields in the reference list display.
New additional reference types including Ancient Text, Dictionary, Encyclopedia and Grant.
Electronic Source is renamed Web Page.
New import filters, connections and output styles. The main one added is the Newcastle Library Catalogue.
Hide empty fields to reduce type scrolling through references.
Reference type lists are now in alphabetical order making locating easier.
New—compressed library option creates a single-file
backup for moving EndNote libraries between computers
Store up to 45 links per reference in the “Link to PDF” field
Convert existing PDF links to the new EndNote-relative link in
one step
Use the PDF icon in the EndNote toolbar to open the first
linked file in the highlighted reference
“Change Text” and “Change Fields” include a new option for tab
and carriage returns used in phrases
Search fields and words using “begins with” and “ends with”—replaces
“Use Full Text Index” of previous versions
“RTF Document Scan” is renamed to “Format Paper”
Customise library and reference toolbars in Mac
OS X version for quick access to frequently used commands
Identify individual references easily in
Spotlight Mac OS X Tiger search results—more descriptive
details available.

Taken from http://www.endnote.com/enXinfo.asp