You don't need bosses for that, regular enemies do fine.
Of course then you do have people who go all "level scale them! I don't want to feel like I've made any progression!!!"
Don't know how you jumped from "No bosses!" to "Level scaling". One thing that regular enemies can't deliver is the feeling of an personal one-on-one-fight, between two worthy opponents. A good boss has basically the same skills as you, but he is a cheating bastard (because real KI wouldn't be implemented in an video game, let's be honest) and you win trough skill. And when you have an reason to hate, it's more delicious and like some already have written, you get an personal connection with him, if the writing is good.
@madscientist
The problem with your examples is, that this was part of the story and not part of the gameplay. A good boss fight is something like the fight against ... let's take The End from Metal Gear Solid 3. Because it's all about skill and knowing the territory. Your examples were all part of the script, but it wasn't you, that had the ideas.
