Here's a simple way to look at it:
Would you rather spend time planning your vacation BEFORE you're in the midst of it, or take time out from your ongoing vacation to plan it?
Do you want to decide on some restaurants that interest you while you're at home with a computer in your lap and can browse all the menus, or have to choose quickly from the "leftover" restaurants that most people aren't that interested in?
It takes some work, but it is TOTALLY WORTH IT to make at least a few ADRs. They don't "tie you down" because you can cancel them if you decide to make other arrangements. They're insurance, and well worth it. Pick a few places that interest you and book them. If you don't "feel like" eating at that place that night, cancel them and do something else. Easy as pie.
I for one will spend my vacation eating at the places I want to eat at, rather than the ones that are leftover after all the other vacationers have picked.
To answer the OP's direct question: Without any ADRs, you will not go hungry. You probably will not be impressed with the food either.
Edited to say: Sorry if this sounds harsh. I am not trying to be a jerk, but I AM trying to give the best and most persuasive advice I can possibly give. I know making sense of the vast sea of restaurants at WDW is hard. For our first trip, I think I only made 3 ADRs because that was all I could process. The UG is a GREAT resource. So are the menus on this site and on
www.allears.net. These boards are a great resource. Advance planning is well worth it when it comes to a WDW vacation.