This doesn't really make bootstrap "flat". Removing rounded corners doesn't make it flat. Removing shadows (and depth) is the main issue. You should be removing

This doesn't really make bootstrap "flat". Removing rounded corners doesn't make it flat. Removing shadows (and depth) is the main issue. You should be removing borders too, right? Even still, "flat" design isn't right for everything. Design should be evaluated for the specific application. The design of bootstrap now is safer than a "flat"-er option, IMO.