The [mtg:cardname] card tags don't show up in (JavaScript?) post previews. All that's visible is the plain text.
I asked the developers about this one: changes to the live post preview would be non-trivial, and this specific fix is pretty low on their List of Projects (by Priority). It may be done later on, but please don't expect a change in the near future.
Fortunately for all of us, there's a five-minute window of free editing -- stuff like this is what that's perfect for!
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This isn't the first issue with the real rendered page being different than the preview. Why do they not use the same engine? It seems a lot more work to have two different renderers than it does to just have one. – corsiKa Oct 1 '12 at 22:31
