Google Instant Search is a very interesting experiment that could change the way you find the information on the Web, but the new interface comes with a lot of unpleasant side effects.
1. Google Suggest can no longer be disabled. Even if you disable Google Instant, suggestions will still be displayed.
2. You can no longer change the number of results. The interface would be less fluid if Google had to fetch 50 or 100 results every time you change your query.
3. Google Suggest no longer shows previously typed searches from the Google Web History. This was a great feature for the repeated queries. Marissa Mayer said last year that "40% of searches on any given day are repeat searches for a user".
4. Google Suggest no longer shows 10 suggestions. Because of the space constraints, Google only displays 4 suggestions, followed by your original query.
5. The fade-in animation has been removed. This feature used to hide the navigation bar and all the links from the homepage until you moved your mouse.
6. "Search within results" is no longer available. This feature has never been useful because it didn't actually search within results. It only allowed you to add new keywords to an existing query.
7. Google's search box is not displayed at the bottom of the page. As Amit Agarwal points out, you have to scroll to the top of the page to change the query.