The ONLY problem (rust really isn't a problem...) with steel for frame materials is that it is "density" challenged. For given tube dimensions (diameter, thickness, etc...) it is indeed extraordinarily strong, but very heavy. That is you can make a much thicker titanium tube that is as strong as a steel tube (which will be much much thinner) and weighs less, due to titanium's density advantage. Same with Carbon and aluminum. Having said that, modern steel frames are quite light. For example for my frame size, 52cm, I was quoted an approximate weight of 2.9lbs for a compact frame made from Columbus Spirit steel tubing, which is highly shaped and butted. The thinnest portions of these tubes are less than 0.5mm. A similar frame of titanium might be 2.6-2.8lbs and maybe 2.2-2.5 in carbon (just guesses). A highly durable, very stiff steel frame can easily be in the 3-3.5 pound range, with which one could have a 16-17lb total bike. You can get even lighter with Ti or carbon, though.