Traction, if defined as getting interest or sales from customers is extremely important. Lots of people think that they may be able to get investors in just on an in idea. The truth is this almost never happens for 1st time entrepreneurs. On the question of major milestones however, I would not worry about this as much. The important thing is to continue to re-evaluate your assumptions, target audiences, product to market need fit. You will find your milestones keep shifting that that is perfectly fine as long as you are getting traction or on your way to getting traction.