Some Panes don't work well with preview, usually because they depend on some context for the preview to work. For example, there are Blocks that will only work when placed on a node page (NOTE: I'm talking about Blocks, not Panes, because a Pane may have the context available to it when using the IPE on an actual node) and so the preview is always blank. It'd be really cool to have a way to just say, "don't try to render a preview for this list of panes."