If you had to rebuild your product from scratch, what would you do differently?

If I had to rebuild my product, I definitely should have figured out something important but wrong with what we have built. Therefore I will self reflect on how the mistake happened in my mind and keep doing what is right until the point I need to accommodate the change to avoid the important mistake. Make sense? For example, if I forget to apply for a trademark or chose a wrong word, which could be critical, I will have to rebrand my product and we'll do the same but will use a trademarkable words as the brand name and apply for the trademark protection once the product launched.

I'd revisit the core user flow first.

noope. always building with the same impact

I'd validate my ideas better and talk to possible customers, and build better user experience in general for the products. Though I've improved a lot in this area lately. But still a long way to go.

Integrate internationalization and oauth much sooner. Easier to do on a new codebase, and VERY painful to do on a more mature live product.

if the product is aimed at different contries, first get to know their cultures and redesign the logo (we are building visual product, so the logo is essential)

Definitely product positioning.

Depends on what kind of product.