From what I was told at Dan Perkins - L level services are the same as P level services but they dont hook up the computer to read anything off the car. If you car is in warrenty, having an L level service invalidates it as to keep the warrenty they have to read the codes (if there are any!) at every service. The reason its cheaper to get an L level service is that its not the official Nissan one so they dont have to pay through the nose for the bits and they dont have to pay for the computer usage which I was told they are charged for.
When I compared the DP L level to P level, you got more in the L level, so they must be saving money somewhere to allow them to do it. L levels services have always been fine on my car, no probs to date.