So glad only a couple of people have tried to light the os/device war flames...

As mentioned already, manufacture customization of the UI can lead to a poor user interaction with a device. This often leads to confusion and bashing of the Android OS as 'terrible' 'confusing' or 'worthless'. The OS it self is wonderful, stable, kittens, rainbows and puppy dog kisses
Just some manufactures have a weird sense of what people want.
Take the often bashed and hated 'Moto Blur' or the despised HTC 'Sense" UIs. Those companies feel/felt their choice in UIs were the best for their devices/customer base. Many of their customers agree and still many disagree and change the UI to suite their needs.
The stock Android UI is rather boring and needs dressed up a little Which is why we have at-least 6 'home' replacement apps' with dozens if not hundreds of themes, plug-ins, etc available.
My choice in launchers/home app is The GO Launcher EX

ETA:
Even in the droid world there is 'fandroids' within 'fandroids' of certain 'ROMs' Just mention Cyanogen(CM7) or MIUI.