All the resizable icons on your homescreen are blur, as are your unified networking inbox (that twitter/facebook/email/sms ***********.) A lot of other things have been rewritten for blur as well - for example, your dialer contains elements of it, your contacts window is different from stock android, the three buttons along the bottom of the home screen are put there by blur (and the weird 7 home screen configuration, as well.) Also, the recolouring of your interface elements and car dock interface also appear to be blur-specific.
Anything that causes the X to deviate from the stock android experience is most likely blur, compare the UI on the Droid X to a moto droid running 2.1 stock and you'll basically be able to pick out the parts pretty easily.
Honestly, I threw launcherpro on it and just choose not to use most of the Blur UI elements. The ones that do work their way into the parts of the phone I actually use I like a -lot.- Haven't seen a performance drain/unreliability issue caused by it, so I assume people are just whining for the sake of it like they always do. There are a couple minor blur-related annoyances I've run across but overall, I've actually found the little additions it makes to the UI quite nice.