Someone on the team must be skilled not only in making things pretty, but in making them usable as well. Without a good UX/visual design, you may as well not even bother.
Once you have an idea of what you're creating, you need to figure out how to make it happen. That's where the Architect comes in. (...) The role of the Architect is to choose the best tools for the job, and to decide when new tools need to be created.
The role of the Developer is to turn the wishes of the Designer into reality while staying within the constraints that the Architect has put forth.
GitHub benefitted from being started by three very well respected and connected members of the rails community. It also was alone in an unaddressed niche. Combined with a great product, this has allowed them to spread explosively throughout the tech world.
Most startups won't have these advantages. Building a great product is important, but even more important is to build a good customer acquisition model. If you can't acquire customers for less money than they bring you, your startup will fail, no matter how good your product.
So, along with your designer, your architect, and your developer, don't forget the sales and business development guys.
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