What's the most important lesson you've learned about building a successful product?

I'm a Product Designer and I've worked at 5+ different start ups that built and launched successful products. I'd say the biggest lesson is that you will find the right features to build from outside your company, not within it. That is to say, talk to your users/customers/target audience. Don't sit in team meetings and guess what your users want. Then go out and test your assumptions and validate them.