The technique you describe is called "No-chill" (see question 9699 and 5374) and is used by lots of homebrewers. Usually the hot wort is placed in a sealed food grade jerry can and allowed to cool overnight. Since the container is sealed and the boiling wort has heat sanitized the inside, the wort should store for months without spoiling and thus beer can be fermented whenever you get around to it.
Beware of using the air lock during cooling as the water in the air lock will probably get sucked back into the wort as it cools.
The main difference between a slow and fast cool is the hop utilization. The aroma hops (last 10 mins of boil) will stay in a hot solution longer during no chill and probably be utilized more like flavor hops (20-45 mins) so the final product may have less hop nose. Rapid chilling ensures that the last hop addition will contribute to the hoppy aroma.
There are many more no chill questions and answers on homebrew.stackexchange.com