Every device has a different vibrate strength. I'm not sure what it is. It could be the differences in individual vibe motors, but I've found that every device is different. That doesn't mean there isn't something wrong with it. I once had an 8320 Curve that would vibrate stronger if it were held upside down and very weak when held right side up. The vibe motor went bad. It happens.
The vibrate is weaker compared to my Curve, but the Curve vibrate would vary in strength depending on the OS version that was installed. The OS controls how many RPMs the vibrate motor turns, so a future update may increase it and make it stronger (or make it weaker :( ).
I TOTALLY agree that it should be user selectable.