I’ve been quite strongly against the idea of having a Thread Description implemented into MyBB.
Some people are already having a hard time writing a good Thread Subject, I don’t think that having a Thread Description would make it any better. On the forums I’ve seen thread descriptions enabled, the only memory I have of those are either repeats of the thread subject, or just useless text such as “help me”, and that for 80% of the threads, the author was too lazy to add a description.
From my perspective, this “feature” isn’t really used, or it’s rarely used correctly. The thread subject can allow up to 255 characters, so any normal thread subject should fit. Those are my 2-cents on why I don’t want this feature to be implemented. 
In my opinion, the thread subject should be short and snappy (like a newspaper heading) to attract attention. The description can then add detail about that thread.
When I used it on my forum it was well used by everyone, and I didn’t see many cases of “help me” etc…
It’ll be nice one to have it as a feature..
I agree with Dennis and DrPoodle.
There is an area for subjects, be to the point and keep it short.
There is an area for the general post, where you give more details.