Oh, I see what you mean now that I tried it. It jumps you out of the title field to the main body of the text. Maybe the forum doesn’t want people making bold text titles. Maybe it’s a consistency thing.
Edit: a ha! I think I just realized why: Threads with bold titles mean you have not read it yet. Threads with normal titles mean there is nothing new that you haven’t read. Therefore, we do not have the option to bold our titles. 😉
You could go with your computer's Character Map.
Or, those alphabet letters that are common to Western European languages and available in Windows. Perhaps it would be a little surprising if you used an Asian writing system for most members. It wouldn't be bold but there would be a certain drama .
A little "addition" would be the suits of playing cards: ♣♦♠♥. Hey now, those are fairly bold!