I agree with what you are saying here up to a point. "Warriors" have less mechanical distinction in the board game than "Spellcasters" do. Because spellcasters have an entire third of a book granting possible game mechanics they can select from and add to their sheet. The Feats section of the book does not in any way compare to the Spells section in terms of variance and width of options, but it does grant
something additional to Warriors that they otherwise wouldn't have. So if you do not use Feats, the warrior characters have less game mechanics to use during play. I agree with that.
The part I don't agree with is when you attribute this mechanical distinction to "defining your character", because your character is all in how you play it. An awesome roleplayer can take an AD&D Fighter with almost no game mechanics and make it more compelling and interesting and useful of a character than a bad roleplayer using the full suite of Pathfinder options to bust out the numbers on the sheet nine ways to Sunday.