It depends on what I am looking for in a trip.
When I was a travel arranger for my last company, I liked to be able to pick from all sorts of flight options to find the ones that best suit my travellers time needs. Then the ability to pick their exact seat on the plane (delta offered this :) ) when actually booking the trip was fantastic since everyone on my team of 200+ had their own preference for seating - aisle within the states, window internationally, wing, front, back, etc.
The ability to find hotels close to a specific address is also very important, especially to business travellers.
Usually I would go to the rental car site directly, however, because of emerald isle status and such it was difficult to book cars through most travel sites.
Some sites offer the ability to book show tickets or other event tickets when you book your trip. However, I've often found that this wasn't very useful as you can get even further reduced cost tickets many times at your destination.
Links to area restaurant options would be helpful just to know whats around.
And if in the US, seriously - on a personal vacation trip lasting more than a couple of days - the ability to find local retail stores (i.e. Walmart) is a godsend. :)