A very complete review about google scholar. They also provide the reader with a nice list of potential state-of-the-art improvements:

Google, Inc. has the intellectual and financial resources (and the largest group of cheerleaders) to create a superb resource discovery tool of scholarly publications. It needs to: a. exploit the highly structured and tagged Web pages with rich metadata readily available in the digital archives of most of the scholarly publishers b. create field-specific indexes for many distinct data elements c. offer an advanced menu with pull-down menus for limiting the search by publisher, journal, document type, publication year, etc. d. consolidate cited references through the ever increasing DOI registry e. collect information of all the relevant materials from the publishers' archive f. develop utilities that enable libraries to launch a known-item federated search in the full-text aggregators' databases licensed by the library in order to check if any have the document from a journal that is not licensed digitally from the publisher