Lou Reed wrote this line (my favorite lyric ever) in the context of relationships (I think). But it's much more applicable than that.
Ideas for new businesses are amazing, wonderful, creative things. What makes ideas so interesting is that they are unbounded by limits - they operate solely in the realm and palette of a person's mind. As far and wide as one can imagine, one can develop an idea for a new business that solves a huge problem, that scales infinitely, that is wildly profitable. The possibilities are limitless.
And therein lies the problem with new application ideas and expressions - they have no limits. They are too broad, too creative. Ideas are too good.
It's what comes next that is so much more interesting - the point (the "lifetime") between taking that idea for a new application or service and actually expressing it in a real form. Building it. Prototyping it. That's when your ideas are subject to limits (technical, execution, market, financial) and those limits actually test your theses and tenets. Prototyping shows you where your assumptions were wrong, maybe, and how your idea may be even better than you thought. Most importantly, it subjects your idea to numerous unanticipated constraints.
I was with a friend the other day who is planning to build an awesome new service that I would use and pay for - it solves a real problem, it scales, the market is right for it. I desperately want him to prototype that service - I want him to see where his thinking is off, how some user/testers will actually use it, how web technology can't scale to this problem. I want him to know where he is right and wrong in his thinking.
This step - the expression step - is just that, a step. It's interim, it's likely to never see the light of day. But it is the most important, and the hardest one.
Because it takes something limitless and subjects it to real life constraints, the point between idea and execution can last a lifetime. But without moving onto the expression, there is nothing.