Unlike an LMS where students must leave the content area to engage in discussion via a Discussion Forum, wikispaces (by default) provides a discussion area behind each page.
This enables you to outline an activity, or pose a problem, on the page and ask students to respond by clicking on the Discussion link in the top right corner of the page.
Students are able to reply to topics/threads, or create their own.

Wiki-wide discussion

It's also possible to adjust the discussion settings so there is ONE only discussion space.
To do this go to MANAGE WIKI - SETTINGS - WIKI INFO - DISCUSSION SETTINGS, and select one discussion page for the whole wiki from the options.
You can also turn off the discussion function fully here.

Adding rich media to a discussion post

For Youtube videos simply paste the following ...
[[media type="youtube" key="-dnL00TdmLY" height="100" width="200"]]
into the discussion post box. The 'key' comes from the video's unique URL in Youtube etc. The height and width can be adjusted.