What makes the difference with Guinness is how far the taps and the pipes are kept clean - well, it makes the difference with all beer of course, but the thing with Guinness is, since no other skills are really needed in serving it (apart from knowing how to pour it, which is tricky enough), the normal skills of looking after beer can be lacking in people selling it. They can get away without them, most of the time. But if the taps aren't clean, or worst of all, if the cleaning stuff is allower to remain in the system, it can taste absolutely disgusting.

As Dylan might have sung:

One kind favour I ask of you,
See that my taps are kept clean