Venn diagrams for analyzing which PPC keywords you and your competitors are both bidding on. The data is incredibly useful and the diagrams are great to utilize during client presentations.
Stellar user experience: The site is not bogged down by too many unnecessary bells and whistles. It's a very easy tool to pick up and start using without requiring much training.
SpyFu also gives you a well rounded snapshot of the PPC Ad copy for your and your competitors. This can be extremely useful to better understand the variations in strategy that different companies are using to achieve their objectives.
SpyFu could use a bit of a visual revamp. I believe that if it offered more of a visually pleasing aesthetic, people would use it more often.
It would be great for SpyFu to develop an alert system that's triggered by major events or changes in your competitors keyword strategy (keyword type, bids, etc), or even if can identify when competitors are adding or implementing major edits to their ad copy.
What differentiates SpyFu from its competitors is that it's simply easier to use and a lot more affordable. At the end of the day, companies will spend a lot of money for other tools that offer a great deal of functionality that no one ever uses.