I hope that this is a constructive suggestion.

For topics that are locked because of content problems- use LOCK icon

For topics that that are locked because of
wrong forum/move to different forum- how about using a different icon (like moving truck??!)

For topics in CC that are locked due to successful transactions- how about using a different icon (like a dollar sign?)

I think more relevant icons would make it easier for everyone to understand the INTENT behind the locked threads, since the 'lock' icon implies something bad.