If you are a heavy user of gmail, you mail have missed the ability of scanning attached files against virus. In a web infested of virus that arrive everyday by email this is a must have, and other free services such as yahoo have provided this feature for already some time.
The only reason why this was not so bad with gmail, is that it has a very decent system to catch unwanted message, including spyware and other beasts. Now, gmail closes the cycle by adding virus protection to its email users. It is a welcome feature that will certainly make the gmail users even more productive with their email reader.