This excellent article by Chris Dixon presents a common strategy for new entrants to establish a market presence that they can build on. The strategy consists of focusing on one essential feature that's missing from the competition (e.g. Posterous' post-by-email), using that to acquire initial customers, and then building that "it's a feature, not a product" initial offering into a full-fledged product.
If that's your plan from the start, it's probably a good idea. Creating a thin wedge with no vision for how it might become a full product is a lot more risky.
Critics sometimes confuse wedge features with final products. For example, some argue that mobile photo sharing is "just a feature," or that game mechanics on geo apps like Foursquare are just faddish "toys." Some go so far as to argue that the tech startup world as a whole is going through a phase of just building "dinky" features and companies. Perhaps some startups have no plan and really are just building features, likely with the hope of flipping themselves to larger companies. Good startups, however, think about the whole wedge from the start. They build an initial user base with simple features and then quickly iterate to create products that are enduringly useful, thereby creating companies that have stand-alone, defensible value.
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