Once a thread gets too long... it becomes an administration nightmare...
For instance, on SRK, there is a thread with 32,000 replies. It is impossible to do any admin functions on it. It can't be locked, moved, approved, etc... because the amount of memory required to process it is outside the scope of the php memory limit. It would be nice if we can simply set "auto lock after 10,000 replies", so that issues like this aren't issues anymore.