Marketing: Something that we promise to have more of than our competitors, but you can't measure it and even if you could you'll find that our guarantee only allows you to start proceedings against us - not any sort of automatic restitution.
CommonSense?: Sometimes our service is crappy, sometimes it's not - that intuitive measurement is a measurement of the quality of the service. SeeAlso marketing...
Technical: When our system says you can have, say, three emails a day (providing no two are within an hour of each other) then we mean it - we have allocated enough resource to fill that requirement.
Although it sounds nice, and it is, such a system (of automatically guaranteeing bandwidth or other resources) simply shifts the problem forward. Instead of finding, halfway through your transfer, that it is going really slowly, you find that your transfer will not even start. Of course, for some things that's better. Full technical discussions can happen somewhere else. But QoS? also encompasses TrafficPrioritisation?, QueueingPolicies? and such like. It's a fact that most network traffic is bursty, so it is useful to be able to utilitise the available bandwidth intelligently. --B
That's just implementation details (and, unfortunately, specifically IP QoS? has hijacked the general QoS? term) - as long as you supply the service you guarantee (and have a system for rejecting requests that you cannot guarantee) then you're providing QoS? and it doen't matter whether you do it sensibly (as you suggest) or by simply having a pipe as wide as the moon. --Vitenka