Here is a more simple understanding: the tubesets are marketed both as an alloy of some strength, and a particular shape/butting profile. It's not simple round tubes any more. Just like you can take sheet steel and form it into a stiff structure like a car, or a washing machine, you can take a thin-wall tubing and shape it in some way that it's stiff where it wants to be and resists torsion well, you get a bike that is uber-stiff but has a nice ride that is described as, "liveliness."
When tubes are welded together, the area that was heated has been affected, normally weakened. The newer alloys are more resistant to heating and are not as affected negatively by welding. This allows a maker to save money since the frame does not have to be heat treated. Much of this metallurgy is esoteric. Because it is such an esoteric subject, marketing has made a mess of everything, lending all kinds of market-speak claims about their products.
The direction steel tube makers is going in is to offer shaped tubesets that are specific for the stress in that part of the frame. The effort is to make the bike strong enough, yet light enough to compete with aluminum bikes and other materials. There is a downside to these ultra-strong, yet thin-walled tube sets: they can be dented more easily. You just have to understand that, in steel, going under 2.5lbs for a frame weight starts getting you into that beer can thin tubing. (I'm exagerating since even the thinnest steel bike tubing is much thicker than that used in beer cans).
What you can easily do with an entire bike is accept more weight in a frame, and take it away in lighter wheel sets, stems, seat posts, forks and bars. As a complete bike, steel can compete favorably with any other popular bike frame material. One thing I will say, it is true that there is a feel to a steel bike. Some people can feel it obviously, others are, "feel-deaf," in that they can't tell the diference between a steel bike and an aluminim bike of the same dimensions and fit.
I will end with this. Too much focus is spent on the material and not enough on the execution and everything else that makes a bike an ergonomic machine. Who cares how uber-light the frame is if you sit on a brick for a seat or ride with fork so flexible going fast is dangerously squirrely. Wheels and tires make a huge difference in bike feel. Total weight is affected by so many more things that the basic frame weight is a wash.
