As a former iPhone 3G owner who suffered through the first 6 months of iOS 4, I am sensitive to the idea of keeping features disabled if they'll significantly affect the user experience.
That said, I agree with the OP. And I'd like to add that it is absolutely ridiculous that the iPhone 3GS will be getting iOS 6 (even in a feature-gutted form), but the first generation iPad is being left behind completely. What the hell is up with that?
jlamb0, I agree with you...I would never want a version of iOS or features that my phone couldn't handle. It wouldn't be worth it to load it with features that are just going to kill the phone, and this is an argument some have about the new upgrade. However, apps and jailbreaking (like everyone says to do) proves that the 4 CAN handle navigation. Maybe not the 3d modeling...that's fine, keep it...I don't need or want that. Same with siri. Turn by turn navigation, I feel, is a feature that should be (and could easily be) added to the 3gs and up.