My Firefox install has seven search engines by default in the "one-click search engine" options. By contrast, DuckDuckGo has 13000 bang shortcuts. Sure, you don't need all of them, but I use dozens: some of them almost daily (like !w for Wikipedia and !wt for Wiktionary), some sporadically (like !tw for Twitter or !a for Amazon) and some of them only rarely (e.g. I picked up a Pokémon game recently and consequently have used !bulba to quickly search Bulbapedia, a Pokémon fan wiki.) Of course I can add all those to Firefox, but DuckDuckGo already has them set up, and has many more that I can use without thinking about them. I think that still counts as a killer feature.