There are several HTC customizations throughout the software - the SenseUI everyone is familiar with is called 'Rosie' - this is the home screen replacement, just like aHome or OpenHome.
The other portions - such as contact integration, the camera and calendar apps, etc - are a bit more difficult to 'turn off', since they've been dropped in as replacements for the stock software.
The only way to truly get rid of HTC's customizations would be to root the phone and go with a stock-android ROM (which hasn't been packaged by anyone yet)