I find the WYSIWYG editor very hard to use. Its slow, inserting and editing code is a real pain, it has happened it screws up line breaks, etc. There is a reason a lot of forum has moved away from WYSIWYG in recent years, especially in tech-oriented forums.
Considering the target audience of the forum, I assume most posters are familiar with markdown, and I think it would be a great addition.
There is of course the option of keeping both (and the icons can of course remain even in markdown mode).
I want to emphasise that my main pet peeve is source code. It seems to depend on server load (which in general is really slow) and each time I want to edit it easily takes 5s to load the editor. And once in the editor it doesn't really provide any features. It only allows me to adjust how its displayed (i.e. monospaced, syntax highlight and boxed).
I usually use my mail client to write a text and then copy and past it here.