Well, I just came from a newly implemented Discourse site which is replacing two legacy sites. We are still tuning it and one of the first things I stumbled upon was not finding a preview. I mean, every sensible site for posting text has some kind of a preview and I was frantically searching for some button to reveal it. I dare say I’m a seasoned forum user, but it took me minutes to come up with the idea to close the yellow and subsequent blue (similar topics) panels.
We often have users that paste long pieces of code or malformatted URLs. A preview is needed already and especially for first time users. Please treat this as user feedback and not as just some abstruse idea.
An (I guess) easy fix is to just narrow the welcome panel down to 2/3 its width to make the underlying preview pane visible. Then it becomes clear that closing the panel will actually achieve something.