The upgrade from 2.0 to 2.1 is a minor revision upgrade. Most users will not notice many of the enhancements besides the obvious LiveWallpapers. New language support, emulator skins(developers), some UI stuff such as fading scroll bars when not scrolling, and other developer related enhancements will not be noticed by most end users.
The developer may not even use some of these enhancements in which case you would see no noticeable changes. Such as the upgraded signal level methods. Sometimes upgrades to software are more noticeable relative to what your upgrading from. Hence minor revision 2.0/2.1 and major revision 1.6/2.0.
EDIT: rooted phones and those with HTC sense UI have more than 3 homescreens.