Not All iPhones Will Get iPhone OS 4.0 Features

Because people are homebrewing updates.

They ARE Compatible, but the coders do not have access to the drivers needed to make everything working correctly. HTC and Samsung have this.

The droid, moment, hero, and n1 use different DRIVERS Because they are different phones. You can't just copy/paste an OS into a device. The only reason you can do this with windows (or other desktop OS's) is because those parts makers release drivers for each update (or the OS writes them in) this is not currently how mobile devices work.

Blackberry, Winmo, android, and yes, EVEN the iphone require a specific version of the OS for each device, The only reason you don't see this with the iphone is since the produce both the hardware and the software, and they release only ONE os update a year, they can push everything on the same page. This is a benefit of single device makers, but it isn't something that makes it inherently superior.