I'm a software guy so the approach I tend towards is to build something which does at least some core functionality first before telling anyone about it, and refine it through usage.
But recently I met someone who was from a marketing background and he had launched an online "coming soon" type campaign which was quite flashy and looked great but there isn't a line of code written or even any software design behind it. I was shocked that he would expose his ideas before having something done because anyone out there could copy the concept and beat him to market. However he does have the advantage of refining the concept through feedback before even investing time in building anything?
What are the pros and cons of the two vastly different approaches to launching a web application concept?