I'm trying to understand.

back in the 70s, the 747 was toted to be this monster place to seat so many and have such a great interior, pianos, bars, etc.

throw in a gas crisis and they packed it with seats and that was that.

now the A 380 comes out in a similar fashion, similar climate and, well, how will it be "better"?

I mean. For example, lets say BA has 7 flights LHR-JFK a day. And assuming the planes used are mostly 747s with a 777 thrown in. Most of those flights, on a given day, are not full.

So why do we think a bigger a/c will do the difference? Arent we just looking at more empty seats?

It will make a big difference on routes where wide-bodies are already full. There are a bazillion of these in Asia, quite a few in Europe, fewer in North America - which is why, aside from regional rivalries, it's usually North Americans who don't see its value. That pattern also matches, roughly, who's buying it. Aside from the economies, many airports are landing-slot constrained, so larger aircraft are the only way to move more people in and out.

Aside from that, there are days when all those BA flights, as well as everyone else's, are full. Since it's not practical to switch aircraft the day before based on how many tickets you've sold, you have to fly something big enough for most days. That's an oversimplification, but the fact that 747s have some empty seats much of the time is not by itself a basis for concluding that an A380's capacity would be wasted.