Touchwiz adds the dock which you cannot remove, where as with the Evo there is no such thing. The icons are also not user replaceable, so you're stuck with what Samsung delivers, unlike Launcher Pro.
In addition, Samsung added a "screen indicator" showing you which screen you're on. It takes up almost an entire row of space, whereas on Launcher Pro the "screen indicator" is a flat bar near the bottom that takes up almost zero space.
So you're able to add less widgets, icons, etc, to the home screen, due to these two factors. The screen is already smaller than the Evo to begin with, and this makes the free space even smaller compared to the Evo.
I just don't like Touchwiz all that much to begin with, but these two negatives and I really hate Touchwiz. Yes you can add Launcher Pro on top, but then you're running 2 home screens on top of native Android, which further slows the phone down. Completely unnecessary.
Also I hate the "iPhone" icon feel of the apps, and when you view the apps, you scroll left to right, not the vertical that comes with default Android. It feels bizarre. This coming from someone who also has the iPhone... I just don't like it on Android.