Actually, as of September 2010 I don't know a firm answer to that question, but I think that the contemplation that leads to the hypothesis is probably inspiring.
Suppose we compare passenger cars that fit 2 to 5 passengers. Almost all of them have a staring wheel, 4 tyres, they're more or less of the same size and get the passengers from point A to point B. Usually they all get the job done, technically speaking, they're practically all the same, just switch the car on and drive. In an average non-highway commuting route the cars have roughly the same speed. Probably the skills of their development teams are also equal: an engineer from one of the companies can probably move to another company and be able to handle the job after about a year of learning.
However, obviously, there is a very big difference between Fiat, Mercedes, BMV, Toyota and Ferrari, despite the fact that they're all just cars that take roughly the same amount of fuel and get their passengers from point A to point B.
My hypothesis is that one way to differentiate something is to differentiate it in style, bias, not functionality or technically fundamental properties(i.e. a flying car or an amphibious car). There are different ways of achieving the same goal and there's always the matter of taste. It doesn't make sense to argue that apple juice is somehow better, more innovative, more effective, more functional, etc., than pear juice, but the two definitely differ and people have their preferences.
If the hypothesis held, the conclusion for me would be that even though I'm just an "average" software engineer/developer/whatever-nice-name and even though it probably is possible to find some other person that has at least as good or much "better" (whatever that means) skill set than I have, I can differentiate myself by the style of my software. For example, I can create software that solves a common problem that is already solved by my "competitors", but I can target people with certain habits or preferences, just like BMV and Toyota do it with their products.
Actually, it is also extremely simple to come up with totally UNIQUE products, ideas, by taking something that already exists and giving it A BIAS. On can just pick anything one likes and quite randomly pick some field, let's say, gardening, military, medical aid, etc., and combine the two. Probability that a thing like this already exists on the market, is pretty low or if it's not low enough, combine something existent with 2 biases in stead of 1, etc.
scissors -> gardening scissors,
tongs -> nut crackers,
cup -> coffee cup.
What regards to the business success of unique things, then that's probably subject to marketing and trial and error. After all, even the usefulness of cellphones was considered questionable at their early days. At early days of computing very few could imagine, how a house-wife or a teenager uses a computer, a personal one even, or that a granny needs a strong, military grade, cryptography for paying her utility bills from her living room, over a computer network.
For a halve joke I point out that what I've been describing in my current post is in line with the saying that every great scientist stands on the shoulders of giants. (I'm not a scientist, but development work is what software developers do.)
Added in May 2013: "Everything has been Done!