I agree that showing the encryption key in the clear is not a serious security flaw but it's IMHO against best practice.
Passwords and key are usually shown in an obscured form, usually with asterisks, and stored in the user or system keychain. You are absolutely right that the security value of these standard practices should not be overvalued but still … what else within the security framework of CrashPlan is not done in accordance with best practice?
(I assume, BTW, that CrashPlan does not use the system or user keychain on the Mac because it is not a real Mac citizen but a Java-based app. Firefox and Wuala – the latter Java-based too – don't use the user or system keychain either.)