Choosing your tech stack with care is one of the more important choices. You're locking yourself in for years to come and mistakes can be very costly to fix.
Getting transactions and revenues are obviously also important, the whole trouble with doing a start-up is that you have to do so many things right.
If YC has shown us anything, it's that companies can successfully pivot at nearly any point of their lifecycle and still be successful. It's also shown us that they can fail at pivoting and have it be nothing to do with the stack they chose.
If you're worried about lock-in then design for portability. Portability means using open standards rather than open source. The neat thing is that with a well-designed site or service you can switch vendors easily. Cloud is a commodity - who cares who the provider is?
