Just that. It pisses me off no end. No other forum allows this and today I even had a post deleted- alerting others to the fact that it was a zombie thread.
This does not encourage new discourse as people, rightly, point out that it is a zombie thread (sometimes a multiple zombie horde.) and disengage themselves.
If you really want to discuss or comment on an old thread you can link to it easily.
I know this has been repeated time and time again but it kills the boards when old threads are in active or trending.