You can read the announcement on the website for the full list of updates and changes. As an example, there are new document features for ranking:
In LETOR3.0 we added in-link number, out-link number, length of URL, number of slashes in the URL, etc. as new features. Also, we extracted those existing features in all streams (URL, title, anchor and body), while features in some streams are missing in LETOR2.0. Overall, there are 64 features (Table. 2) which can be directly used by learning algorithms.Also of note, the document parser used to index the documents changed (different tf counts) and the definition of some of the document fields differs slightly from version 2.0. Furthermore, the IDF calculation changed significantly.