Please don't forget that, initially anyway, the A380-800 will only be certified to hold 555 passengers. This represents a 35% increase on the seating capacity of the Boeing 747-400 (presumably we are talking about standard 3 class here - the 747 figure is 412 by these numbers). However, the aircraft will have 49% more floor space than the 747-400. Now, for those of you who are not that great with maths, this means the, per passenger, there will be more floor space per passenger on the A380-800 than on the 747-400. Airlines will not be able to do anything about this until the aircraft is certified to carry more than 555 passengers. Now whether or not such features as duty free shops (read cupboard), libraries (read bookshelf) or showers (probably no larger than the size of a toilet cubicle) etc become realities is something we will have to wait and see. But you cannot argue with the figures - the A380-800, when it enters into service in 2006, will have more space per passenger than the 747-400.
As I and others keep saying in other threads, this "49% more floor space" does not represent as much actual usable space per passenger as you are suggesting. Much of it is taken up by the extra aisle, stairway and entrance/exit space needed to satisfy evacuation requirements for both decks.
Since the 747 is (mainly) a single deck aircraft, it is more efficient in using floor space than the 380. It needs only two full length aisles. It does not need a cruise-ship like stairway to board the upper deck. The A380's big stairway is not a luxury feature. It is a design neccesity if you have 2 full decks. It takes up a lot of space.
Someone also mentioned the effect of an accident. Sooner or later it will happen, but chances are by the time it happens the plane will be so well established that it will not be able to be avoided or withdrawn. This is what happened with the 747. Two 747's were involved for what was the worst accidental plane crash in history - when the two planes colided the ground in Tenerife. Only a crash of a full A380 with no survivors could surpass this - yet the 747 did not get a bad reputation even from Tenerife.
One concern that may have some validity is that of in-flight medical diversions. The more passengers you carry, the more risk of a medical diversion - which is the most common kind of unscheduled landing. Such diversions are more expensive, inconvenience more people, and play more havoc with schedules when they happen to larger aircraft. For many obvious reasons, it is better to have a 777 delayed than a A380. From a passenger's point of view, the more fellow passengers you are traveling with at a time, the greater risk that your plane will be diverted. Some have hypothesized that this statistical risk may set a practical upper limit to the passenger capacity of a commercial airliner, making it unlikely that we will ever see such craft as a 1,000 passenger BWB.