I feel that the ability to make named quotes without cluttering the quoted's notification box would be useful, in any case.

And GM powers should never extend to deleting and/or in any form altering the posts of other players - there is a reason most sites pick their mods carefully, and why mods should stay away from topics they feel passionate about. It is very unlikely there wouldn't be griefing and/or other destruction of others' creative content. I also support leaving visible-to-staff logs of the edited (and deleted) posts' original contents (it consumes some extra storage space, but it also generates a means to handle rogue mods and "I-didn't-do-anythings").
Locking a thread? Yeah, I would not allow that, either.