Having to register to perform some basic operations that doesn't really need registration. Or allow a "one-click" registration method.
The user should have the right to decide. I end up with a lot of "one-time use" accounts on so many websites !

Some examples where it shouldn't be necessary :

• downloading files (how many "code" website ask you to register to allow you to download a sample file ?)
• searching (yes, some forums disable search for non registered users)
• accessing an article (when its access is free)
• ...